Thirteen adult patients with nephrotic syndrome resistant to corticosteroid administration were treated with azathioprine daily for four years. Progressive improvement occurred in all 13 patients so that they were in complete remission after one to three years. The onset of apparent improvement was earlier and the rate of progressive improvement more rapid in patients who had selective proteinuria before therapy than in those whose proteinuria was nonselective. One patient who stopped therapy after 20 months suffered relapse but again responded when azathioprine treatment was restarted. Observation after the four-year therapeutic period ranges from three to 15 years; there have been no relapses during this time.