Two men are reported who developed cutaneous fibrosis indistinguishable from that encountered in progressive systemic sclerosis (PSS, scleroderma) after treatment with the antitumor agent, bleomycin. In vitro study of the dermal fibroblasts of 1 patient showed that these cells synthesized increased quantities of collagen and therefore resembled cells derived from individuals with naturally occurring PSS. Bleomycin was shown to stimulate collagen production by cultured normal skin fibroblasts. Also, in 1 of the patients, bleomycin stimulated a lymphoproliferative response in vitro. These effects of bleomycin may be responsible for certain toxic reactions to this drug in the skin and lung.