The diagnosis of multiple myeloma was made in two white men, aged 55 and 59 years. They were treated with cyclophosphamide for 98 and 44 months respectively. Patient 1 also received a nine-month course of combined therapy with melphalan, procarbazine, and prednisone. Both developed acute erythroid leukemia, 98 and 71 months after the original diagnosis of myeloma, and died of subarachnoid hemorrhage and cardiac arrest. Patient 1 developed squamous cell carcinoma of the skin with recurrence, and Patient 2 developed anaplastic carcinoma of the urinary bladder. Palliative radiation therapy was given. The development of erythroid leukemia plus carcinoma in these two men suggests mutagenic change secondary to cyclophosphamide therapy.