A case of cancer of the bladder is reported in a patient treated 5 years previously with cyclophosphamide for membranoproliferative glomerulonephritis. The responsibility of cyclophosphamide for this type of tumour is well documented in the literature and should lead to a limitation of its use in disease with a favourable prognosis and to close surveillance via regular cystoscopy and blind biopsy in patients who receive more than 50 g of the drug.