971 samples of urine and washes of the urinary bladder in 643 patients were investigated cytologically. Urothelial carcinoma was present in 27 patients. In 11 cases the carcinoma was diagnosed by urinary cytology; 7 of the carcinomas were localized in the urinary bladder, 3 in the renal pelvis and 1 in the ureter. By contrast, all 19 tumors diagnosed in washes were localized in the urinary bladder. Diagnostic sensitivity was 100% in urine and about 75% in bladder washes. In 5 other patients cytologic diagnosis of carcinoma could not be confirmed by histology. In 4 of these, urothelial papillomas were present and in 1 patient a carcinoma of the bladder had been resected previously. Thus, the diagnostic accuracy of urinary cytology is good if essential methodological prerequisites are fulfilled. Therefore, urinary cytology appears to be an appropriate method for screening programmes in high risk patients such as phenacetin abusers. The estimated costs of a screening programme in Basel are 50 000-150 000 Swiss francs. However, a preliminary study to verify the value of such a screening programme is suggested.