We describe 46 adults with idiopathic focal segmental glomerular sclerosis (FSGS). The mean age was 36.9 years (range, 15 to 80 years). Males represented 61%, and 65.2% were white. Hypertension was a presenting feature in 63% and 32.6% had microscopic hematuria. Twenty-nine patients had nephrotic proteinuria (greater than or equal to 3.0 g/24 h) at presentation, and 13 had renal insufficiency (serum creatinine concentration greater than 1.5 mg/dl). A mean follow-up of 59.8 months (range, 3 to 255 months) was obtained. In addition to segmental sclerosis, glomerular hyalinosis was observed in 65.3% of biopsies, and this was similar irrespective of the severity of proteinuria. Sixteen of the 29 patients with nephrotic proteinuria received prednisone therapy (60 mg/day) for at least 1 month. Three received cytotoxic agents in addition. A response to therapy was observed in 50%, 5 achieving a complete remission and 3 a partial remission. No patient with non-nephrotic proteinuria received prednisone therapy. The clinical course of each patient was evaluated based on the slope calculated by the linear regression method using the inverse of serum creatinine from the time of presentation to follow-up. Patients with non-nephrotic proteinuria had a better prognosis than nephrotics (P less than .05). Nephrotic patients responding to therapy had a better course than non-responders or patients not treated (P less than 0.01). At the time of last follow-up, 8 patients had progressed to end-stage renal disease, 6 of whom had presented with nephrotic proteinuria. No patient responding to therapy had progressed to end-stage renal disease.(ABSTRACT TRUNCATED AT 250 WORDS)