Reproductive function was studied in 31 adult male patients with Behçet's disease during treatment with cyclophosphamide and/or colchicine for 1 to 64 months. Semen was obtained from 27 patients. Azoospermia or severe oligospermia was found in 13 of 17 patients receiving cyclophosphamide with or without colchicine, whereas the sperm count was almost normal in six patients treated with colchicine alone and in four patients receiving neither drug (control patients). Blood samples were available for 31 patients. The mean follicle-stimulating hormone level among 12 cyclophosphamide-treated patients was significantly higher than that of 6 colchicine-treated patients and 6 control patients. These results indicate that cyclophosphamide impairs spermatogenesis in adult men, whereas colchicine does not.