We investigated phylogenetic relationships within Erythronium using DNA sequences from the chloroplast gene matK and the internal transcribed spacer (ITS) of nuclear ribosomal DNA. Erythronium, Amana, and Tulipa form a strongly supported clade that appears to be of Asian origin. Erythronium is widely distributed in the Northern Hemisphere, and consists of three well supported and geographically distinct clades occurring in western North America, eastern North America, and Eurasia. ITS analyses suggest that the Eurasian and eastern North American clades form a monophyletic group that separated into two lineages following divergence from the western North American clade. Although the three clades are each characterized by distinct morphological features, convergence both among and within geographic areas is seen in several characters. The western North American clade exhibits greater morphological and molecular diversity than either of the other two clades, and many of its species hybridize, suggesting both recent and ongoing speciation. The geographic distribution of Erythronium suggests initial diversification in Tertiary mixed mesophytic forest, followed by fragmentation of the range during the late Tertiary or early Pleistocene and subsequent radiation in western North America.