We studied 100 renal biopsy specimens from adults with the primary nephrotic syndrome in an inner city hospital serving mostly black patients and found that 47 had focal segmental glomerulosclerosis. Most of the men presented in the third decade of life, a peak distribution not seen in women. Half of the patients were hypertensive at presentation. Two thirds of the patients had not used intravenous drugs. The addicts were younger than nonaddicts (mean +/- SD age, 27 +/- 4 years vs 35 +/- 13 years), had greater proteinuria (10 +/- 5 g/d vs 6.3 +/- 5 g/d), and exhibited more glomerulosclerosis and tubulointerstitial fibrosis on biopsy. Of the 18 patients (8 addicts) remaining under our care, 4 addicts and 4 nonaddicts became uremic within 3 years. We conclude that even in the absence of drug addiction, focal segmental glomerulosclerosis is a common cause of primary glomerular disease in black adults, in whom it may represent a nonspecific glomerular reaction to injury. The prognosis in the nonaddict may not be different from that in the addict, but more patients need to be studied.