As a result of inconsistencies in morphological characters, Cerastium pumilum and C. glutinosum have been misunderstood or confused in many European floras since the 1960s. In the second volume of the Flora Nordica, a revised treatment of C. pumilum s.l. is provided and this concept is tested here for eastern Central European populations. The cytometric and morphological part of the study is based on living plants from 85 populations in the Czech Republic, Slovakia, Poland, Austria and Hungary. Flow cytometric analyses of the samples revealed two groups differing in ploidy level and corresponding to two cytotypes (a known octoploid, 2n ≈ 72, for C. glutinosum and yet unknown dodecaploid, 2n ≈ 108, for C. pumilum). Eleven morphological characters were scored or measured in plants of known ploidy level and the data set analysed using multivariate statistics (principal component analysis and canonical discriminant analysis); the two morphologically well-separated groups were identical with the two cytotype groups detected by flow cytometry. Based on these results, we suggest treating the detected cyto-morphotypes as the species C. pumilum and C. glutinosum. Our analysis further revealed that the traditionally used characters (glabrous vs. hairy adaxial surface and presence vs. absence of a scarious margin to the tip of the lowermost bracts) are not taxonomically informative. The characters best differentiating the species include indument on the lowermost vernal internodium, length of mature stylodia, length of glandular hairs on sepals and maximum diameter of mature seed. A key for identification of both species is also provided. A revision of almost 1600 specimens deposited in 16 Central European herbaria revealed that the species show different distribution patterns in Central Europe and partial habitat segregation. Specimens from the Czech Republic previously assigned to C. litigiosum were identified as C. pumilum; consequently, C. litigiosum must be removed from the Czech flora.