What about Rhododendron luteum? Native Eastern American species come in yellow, too.

Jdumas-
Here are the two little leaf rhodos I plant for my gardening clients here in NH Z4-5 when I need a yellow. They sucessfully winter over and thrive. 'Mary F.' is truly yellow. 'Jerhico' is a pale yellow to start and then ages to white; this is OK because if you have bright blooms on other plants coming on, pale yellow can look very washed out. The soft white goes with just about anything. I did plant the little 'Tow Head' once in a rock garden, but it was a disappointing greenish yellow--rather sickly looking. Haven't used that one again.

R. 'Mary Fleming'
http://www.rhododendron.org/descriptionH_new.asp?ID=594

R. 'Jericho'
http://www.rhododendron.org/descriptionH_new.asp?ID=570

The American Rhodo Society lists both of these plants as doing well in Z6; see their state chapter charts below. They have a very useful site.

http://www.rhododendron.org/performers6.htm

Also check out the Northern Lights series of deciduous azaleas--very bold in the early spring landscape, blooms come early before the foliage: 'Lemon', 'Golden', and 'Spicy' are all yellows. I had a treat this year--the Northern Lights we planted in June for a customer, bloomed in September; we saw both the flowers and the foliage together. Extremely lovely.

Ginger

Rh. 'Capistrano' alledgedly should be 'a real yellow' with no pink, orange or salmon tint in it.
I have to see yet actual color of mine, which I bought this year.

Here is a link that might be useful: Capistrano

jdumas;
a hearthy welcome to the Gardenweb and the Azalea/Rhodie
forum;

Each year the Massachussetts Chapter of the American Rhododendron Society submits a 'group' order to Van Veen Nurseries of Portland Oregon, a wholesale grower . These plants are mostly yearlings from rooted cuttings. These are hight quality plants at an exceptional prices. Here is 2005 possible 'yellow' hybrid selection that is available for members of that chapter;
August Moon , Golden Star
Banana flip , Goldfort
Bernstein , Goldstrike
Blondie , Goldsworth Yellow
Bo-Peep , Grumpy
Borneo , Harvest Moon
Cream Crest , Hotei
Crest , Inamorata
Crystal Springs , Jeanne Church
Dr.William Fleming , Merley Cream
Evening Glow , Moonstone
Festivo , Mrs Betty Robertson
Flava , Nancy Evans
Golden Harbinger , Odee Wright
Golden harvest , Shamrock
Unique , Viscy
Yellow Hammer , Yellow Petticoats
Yellow Saucer-,

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This chapter also has a wonderful program called Plants for Members (P4M) where members take cuttings of their unique rhodies that they have in New England area and they propogate those and make them availbale to the other members-- last years cuttings 2004 - the list contained
over 5000 plants that were available- You will 'never' see that many varities in 'any' catolog in the world. The volunteers from the chapter who take care of this function for other members meet in Rhode Island. Even Though I live in Illinois , I took an oppurtunity myself this past spring- in obtaining the latest Hybrids that Mr.Joe Parks created in New Hempshire.-

If they dont grow it there, it probably doesn't exist unless it is very new. Check out the best web site in the world about Rhodies , and especially for zones 4-7.

Here is a link that might be useful: Mass Chapter

Many thanks to Ginger, Ron, Ego45, and Tadeusz for a wealth of helpful information. I was particularly impressed by the vibrant yellow of 'Capistrano', 'Goldkrone' and 'Golden Lights' varieties, but I also like the paler yellow of 'Jericho', 'Goldsworth Yellow' and others. To make the decision harder I really liked the photos of 'Lackamas Blue' that I've seen, too. I'm trying to be consistent with the yellow & blue pairing that I already have in the same general area of the yard. In the spring there are daffodil and forsythia paired with muscari and squill; while in the summer there are yellow daylilies and coreopsis paired with lavender, caryopteris, hibiscus, salvia and veronica, among other things. I was hoping to brighten up the shadier areas with something that contrasts with the white azaleas I already have. It looked so much more colorful this year, considering that the previous owners had just yews and privet, exclusively. But, in fairness, they did add a beautiful pink rhododendron on the other side of the yard that was obviously well cared for. It's fun to plan and research now, and I'm looking forward to more improvements and more color in the years ahead. Thanks again.
-John

I will second that comment on 'Mary Fleming' Rhododendron. I have had one for nearly ten years and it is a super variety. A 'small leaf' variety, by the way. And the deciduous azaleas would be great to look through to pick a yellow. There are many. One I like is 'Lemon Twist' which again I have had for a number of years. I am about equi-distant between Boston and the Cape. Come on down in the spring and I could show you both in bloom and doing very well. 'Hong Kong' I have had for years (a Rhodo) and it is good to fair. Consider membership in the Rhodo Society for much info and as has been pointed out 'group purchases'. We have a nice display of Rhododendrons at the Mass Hort Society site in Wellesley called 'Elm Bank'. I do not know what varieties are growing there but good ones I am sure. Les

A great late summer yellow flowering azalea that I have in my front yard is Weston's Lemon Drop.

I want to thank all of you again for your helpful suggestions. I did end up buying the 'Capistrano' variety and planted it earlier this spring. It really looks wonderful and healthy. Very dense with leaves and tons of buds that are just beginning to open right now. I have lots and lots of galium odoratum planted all around it and the small white blossoms (that are just opening also) really help to make the yellow of the Capistrano stand out. I would highly recommend this variety, and I'm told that it is quite hardy too.

Capistrano is a very nice variety but if you would like a deeper yellow, try Goldkrone. This has been completely hardy here for at least 10 years although it did not bloom fully after the January 04 conditions. Goldkrone likes/needs considerable shade, but it is a deeper, more saturated yellow than any other rhodendron I know of that can be grown in New England.