Among the Corydalis spp. with yellow flowers in Illinois, Slender Corydalis has smaller than average flowers (about 1/3–1/2" long). Only Corydalis flavula (Pale Corydalis) has smaller flowers (about 1/4–1/3"), while the others have larger flowers (about 1/2–1" long). Pale Corydalis also has longer pedicels (1/4–3/4") than Slender Corydalis. The foliage of Slender Corydalis has a similar appearance to other Corydalis spp., except the lobes of its pinnately compound leaves have tips that are more blunt than some of the others. A closely related subspecies is Corydalis micrantha australis (Prairie Corydalis). This latter subspecies has longer seedpods (about 2/3–4/3" long) and longer flowering stalks that extend beyond the foliage. Other Corydalis spp. differ from Slender Corydalis by having constricted or knobby seedpods that are less erect, and some Corydalis spp. have larger seeds (1.5–2.0 mm. across).