We report a case of a 65-year-old man with microscopic polyangiitis who developed prostate cancer and gastric adenocarcinoma after prolonged oral use of cyclophosphamide. Acute hepatitis with jaundice and marked increase in aminotranferases occurred after 6 months of flutamide treatment for metastatic prostate carcinoma. It is suggested that patients with vasculitis or other autoimmune disorders should avoid prolonged use of cyclophosphamide and other cytotoxic drugs in order to minimize long-term adverse effects, of which the risk of cancer is by far the most important. In patients on flutamide treatment, careful monitoring of flutamide administration with repeated liver function tests should be undertaken, and the drug must be immediately discontinued in patients with abnormal results to avoid progression of liver injury.