A follow-up study of 112 patients with advanced renal failure showed a median survival, after serum creatinine concentration exceeded 10 mg/100 ml, of 68 days in 36 patients, and an interval to the first dialysis of 51 days in 76 others. Survival decreased as serum creatinine levels increased and was shorter at any given concentration, if there was no transient decrease in serum level. One peritoneal dialysis increased survival significantly. The interval to the second of repeated dialyses did not differ significantly from survival after one or no dialysis, which was 19%, 9%, and 6% at 1, 2, and 3 years, respectively. Transient improvement in renal function occurred in 20.5% of patients, was achieved in a mean of 34 days, and persisted for a mean of 414 days; it was usually achieved by control of infection, relief of obstruction, or improved renal perfusion. The increment in serum creatinine levels varied from patient to patient and in the same patient. The mean interval required for the serum creatinine concentration to double was 376 days. Survival predictions showed a correlation coefficient of 0.216 and an error of 268 days.