A retrospective study was conducted in 93 patients (55 adults and 38 children) with classical focal segmental glomerulosclerosis drawn from the Toronto Glomerulonephritis Registry. The average follow-up period was 11 years, with a cumulative experience of 1,053 patient-years. Both adults and children were similar in profile at the time of entry, except that the nephrotic syndrome was more common in children (55% of adults v 76% of children; P < 0.05). During evolution of the disease, however, the percentages became very similar with 82% of adults and 89% of children developing nephrotic-range proteinuria. At the last observation point, the outcome of patients (adults v children) was complete remission, 22% versus 42%; end-stage renal disease, 42% versus 34%; chronic renal insufficiency, 13% versus 11%; and persisting abnormality, 24% versus 13%. Although there were more children than adults in complete remission, the rate was equal in the treated adults compared with the treated children (44% v 47%). Although optimal duration of steroid therapy cannot be determined by this review, treatment beyond 6 months does not appear to be beneficial. The best guide to prognosis remains complete remission, since long-term renal survival in both age groups with this event was 100%. Those without a complete remission generally progress, although even at 10 years the survival rate is 62% in adults and 58% in children.