Chromosomal investigations were performed on fibroblast cultures established from tumour tissue of six patients with multiple basal cell carcinoma, and from one patient with a solitary basal cell carcinoma. In four instances, fibroblast cultures from specimens of unaffected skin areas Were examined simultaneously. Metaphases of peripheral blood lymphocytes were analysed in all patients. Three individuals showed increased rates of chromosomal breakage and rearrangement; the possibility of a relationship between these findings and the Occurrence of multiple basal cell carcinoma is discussed:1). The chromosomal aberrations noted in one patient with multiple arsenic‐induced basal cell carcinoma probably reflect the long‐term effect of exposure to arsenic.2). In the second case, the aberrations found in cultures from unaffected skin and blood lymphocytes may be due to repeated X‐ray therapy of multiple nevoid basal cell carcinoma3). In the third patient, likewise affected by multiple nevoid basal cell carcinoma, the etiology of the increased frequency of chromosomally altered cells remains obscure. Taken together with two other observations (Happle et al. 1971, Happle&Kupferschmid 1972), the aberrations could indicate that in some patients with nevoid basal cell carcinoma syndrome the incidence of spontaneous chromosomal breakage tends