Would it be these?

Karen

Here is a link that might be useful: Winter Aconite

Did it look like this?

Here is a link that might be useful: Lesser Celandine

Yellow flowers in wet locations early in the season usually says marsh marigold to me. But if it's not that, can you post a picture? That would help immensely with the ID!

FataMorgana

Thanks everyone! I can't post pix (just got a new digital camera, have yet to learn how to use it!) but with y'all's help I figured out that my mystery flower is, as mistascott suggested, lesser celandine. I was pleased to learn that it is also in the buttercup family, along with the marsh marigold, so I was at least headed in the right direction! Thanks again everyone!

Glad I could help! I looked up Marsh Marigold in my wildflower ID book (National Audubon Society) and Lesser Celandine was listed as often being confused with it. The ivy-like leaves you described had me convinced it was Lesser Celandine.

Just in case you wanted more info, here is what my book says on Lesser Celandine/Pilewort (Ranunculus ficaria): 5-10" height; glossy heart shaped leaves; Flowers March-May; Flowers 1-1.5" wide; 3-4 sepals; 8-10 petals; many pistils and stamens. Prefers lawns, shaded areas, streamsides, and moist undisturbed places.

Also, it is NOT native (European origin) and is considered an invasive groundcover in the U.S.

Here is a link that might be useful: More Info

I'm also in Baltimore City. Almost 99% sure you're talking about Lesser celandine (Ranunculus ficaria). http://en.wikipedia.org/wiki/Lesser_celandine
It goes dormant after blooming, but lives as a bubil until the following Spring. VERY invasive in Baltimore City. Can easily choke out natives. It's easily dug up and separates from the soil readily. Roots are very shallow. Spraying poison on it is ineffective.

Very hard to eradicate. A definite problem plant here in the mid-atlantic states.