In a clinical series of 242 patients with chronic non-obstructive pyelonephritis, 142 patients were classed as chronic abusers of drugs containing phenacetin. Seventy-nine per cent of all the patients were followed up for one to eleven years (average 5.3 years). Eight patients (about 8 per cent) in the group of analgesic abusers developed transitional cell tumors of the renal pelvis, but no such tumors arose in the non-abusers.

In the adjacent surgical department transitional cell tumor was diagnosed in another five analgesic abusers during the past eight years. The group of analgesic abusers with renal pelvic tumor (13 patients) was compared with a group of non-abusers with such tumors (15 patients) from the same surgical department. The mean age of the patients in the first group was lower and females were in the majority, while in the second group elderly men with prostatic hyperplasia predominated.

Tumor differentiation tended to be lower and infiltrative growth more frequent in the analgesic abusers. This may have been due to the later diagnosis in this group. Suggestive circumstantial evidence of a carcinogenic effect by phenacetin-containing drugs is discussed.