A study was made of the frequency of polyploidy in fibroblast cultures from different types of reduction malformations. An increased frequency of tetraploid cells was found in skin cultures from biopsies taken from extremities with unilateral, longitudinal meromelia of unknown origin and from extremities with reduction malformation due to thalidomide embryopathy. The findings were compared with those in cultured fibroblasts from a healthy extremity in the same patient and from skin of non‐malformed patients. Other malformations such as cleft lip and palate, bilateral reduction malformations of the extremities and unilateral terminal, transverse meromelia of unknown origin showed no increase in tetraploidy frequency. The possible pathogenetic importance of the observation is discussed.