In a consecutive series of 88 cases of carcinoma of the kidney and upper urinary tract seen at one hospital, 31 had malignant urothelial tumours of the renal pelvis or ureter. Forty-two per cent of these transitional-cell carcinomas occurred in patients with renal papillary necrosis following upon prolonged and heavy analgesic ingestion. Other possible aetiological factors were heavy cigarette smoking (61% of cases), long standing urinary obstruction or infection (23%) and possible occupational exposure (6%); in only four cases (13%) was there no identifiable aetiological factor. Those cases with analgesic nephropathy were characterised by renal functional impairment, hypertension and interstitial nephritis, but there was no difference in the clinical behaviour or pathological appearances of the tumours in the two groups. The clinical and experimental evidence that certain metabolites of phenacetin are carcinogenic is reviewed.