During the past 11 years, the Metro Toronto Glomerulonephritis Registry has prospectively followed all cases of glomerulonephritis starting from the time of biopsy. Focal segmental glomerulosclerosis was diagnosed by strict histologic criteria in 103 patients. Exclusion of patients with follow-up of less than 12 months reduced the number to 93 (55 adults and 38 children). Mean length of follow-up from the time of biopsy was 61 months. Ninety percent of children, but only 33 percent of adults received treatment with steroids, with or without cytotoxic drugs (p less than 0.001). Complete remission, defined as daily proteinuria of less than 250 mg, was not different in adults (39 percent) from that in children (44 percent), with a mean remission duration for all patients of 38 months. Chronic renal insufficiency, defined as a creatinine clearance of less than 0.8 ml/second/1.73 m2 for more than 12 months, was similar in adults (40 percent) and children (34 percent). Five-year renal actuarial survival, defined as the absence of chronic renal insufficiency, was 96 percent for patients with a history of complete remission, and 55 percent for those without (p less than 0.0002). Logistic regression analysis showed treatment to be the only significant factor for complete remission (p less than 0.001). Complete remission, in turn, was important for renal preservation, defined as the absence of chronic renal insufficiency (p less than 0.001). Age did not affect the treatment response or long-term renal outcome in focal segmental glomerulosclerosis. yet, the percent of adults treated was much lower than that of children, despite the fact that the majority of the untreated adults had the same clinical parameters as the treated adults and children. Thus, a judicious course of treatment is as much indicated in adults as in children with this disorder.