Several reports in animals, and sporadic case reports in humans, have suggested that kidneys with decreased nephron mass may be more susceptible to the development of focal-segmental glomerosclerosis. This prompted a reexamination of our previously reported group of pediatric donor-adult recipient renal transplant combinations. Data were analyzed from 31 adult recipients who had received renal transplants from cadaver pediatric donors (less than 6 years) with graft function for greater than 6 months and no evidence of chronic rejection. These were compared with a control group transplanted during the same period with adult donor kidneys. Immunosuppression consisted of azathioprine/prednisone or quadruple therapy in 16 and 15 patients respectively. End-stage renal disease (ESRD) was secondary to chronic glomerulonephritis (n = 9), diabetes mellitus (n = 6), polycystic kidney disease (n = 5), and miscellaneous causes (n = 11). Twenty patients had radiographic documentation of renal hypertrophy posttransplant. All patients had serial 24-hr urinalysis for protein and creatinine after transplantation during periods of stable renal function. Ten patients had renal biopsies performed at a mean time from transplant to biopsy of 10.4 +/- 1.6 months. Seven recipients had biopsies that revealed glomerulosclerosis at 13 +/- 6 months posttransplant. Protein excretion and serum creatinine in these patients were significantly higher than in control patients (1.6 +/- 0.37 vs. 0.49 +/- 0.15 g/24 hr and 1.96 +/- 0.11 vs. 1.64 +/- 0.09 mg%; P less than 0.03 and P less than 0.01, respectively). Only 3 of 25 control adult donor recipients developed proteinuria greater than 0.8 g/24 hr within 2 years of transplantation vs. 15/31 pediatric donor recipients. No correlations with the etiology of ESRD, age (greater than or less than 40 years), weight, sex, diabetes, hypertension, or the number of acute rejection episodes could be found. Our data suggest that adult recipients of pediatric donor renal transplants may be at greater risk for the development of glomerulosclerosis than those recipients receiving adult donor kidneys.