Nuclear genome size, as measured by flow cytometry with propidium iodide, was used to investigate the relationships within the genus Ficaria Huds. (Ranunculaceae). For the five diploids the genome size (2C) ranges from 12.5 pg to 23.2 pg. The three tetraploids range from 30.1 for F. chrysocephala (P.D.Sell) Zonn. comb. nov. to 36.2 pg for F. fascicularis. Therefore – despite the recent literature considering them often as subspecies – they are here regarded as species. Lesser Celandine (F. verna) as originally described by Linnaeus is probably of Dutch or Swedish provenance. It is a nearly sterile tetraploid with axillary bulbils. For the other species, there seems to be no relation between ploidy and bulbil formation. The amounts of DNA per nucleus suggests that the common tetraploid F. verna is probably an allotetraploid with F. calthifolia and F. ambigua (syn. F. fertilis) as parents. The Ficaria material from Austria (as F. calthifolia) has 15.5 pg for the diploid form and with 31.2 pg precisely double that amount for the tetraploid form (reported as F. verna). The plant from Kazakhstan probably belongs to F. stepporum. The sister genus of Ficaria, Coptidium, has 27.0 pg for a likely triploid. Peculiar is the wide distribution of tetraploid plants that have approximately 30 pg. These plants are generally considered to belong to F. chrysocephala and come from distantly located areas as northern Portugal, Bulgaria, Crete, Serbia and Latvia. In this study, the genome sizes are evaluated in combination with available morphological and geographical data.