Urinary bladder toxicity is a prominent toxic effect of cyclophosphamide. Over 600 months of oral cyclophosphamide has been given to 46 patients with SLE, Wegener's granulomatosis, RA, idiopathic nephrotic syndrome, and hypereosinophilic syndrome. Thirteen (28%) had urinary bladder complications. Three distinct problems were apparent: chronic cystitis in 6, abnormal urine cytology in 3, acute hemorrhagic cystitis in 4.These patterns could not have been segregated without urine cytology. Persistence of abnormal findings may lead to irreversible bladder toxicity.