Fifty-nine allografts were placed in 43 patients with renal failure from focal segmental glomerulosclerosis (FSGS): 27 allografts were put into 16 children aged less than 15 years, and 32 allografts into 27 adolescents and adults. Recurrence of FSGS was noted histologically in 13 allografts, 10 in 8 children and 3 in adults. None of the 9 children and 24 adults who never developed an allograft nephrotic syndrome showed FSGS in their allograft biopsies. The age of onset was a strong risk factor for recurrence: recurrent FSGS developed in 8 of 16 children (50%) but only in 11% of adolescents and adults (3 of 27 patients). Although the time from apparent onset to renal replacement treatment was shorter in those with recurrence than those without in the children, there was no difference in the time spent on dialysis prior to transplantation. Mesangial prominence was observed in the original biopsy in 12 of 13 patients with recurrence, and recurrence rate was similar in living and cadaver donor allografts; class I MHC matching was similar in those with and without recurrence. Three allografts treated with cyclosporin A as well as 9 with azathioprine showed recurrence. Of 9 second or subsequent allografts placed in those with recurrence in the first allograft, only 3 showed further recurrence. In 3 re-grafted after 13, 11 and 5 years, normal function was seen.