Seven patients who had an initial attack of nephrotic syndrome in childhood and had frequent relapses even after cyclophosphamide therapy were given a 2-year course of azathioprine. The mean annual relapse rates decreased from 2.4+/-0.5 in the year preceding azathioprine to 0.4+/-0.8 in the 1st and 2nd years after its initiation. All six patients who were observed for more than 6 months after discontinuation of the therapy were relapse free for this period. Average doses of prednisolone could also be decreased in the 2nd and subsequent years after the therapy. There were no significant toxic effects. Long-term azathioprine therapy may be well tolerated and effective for nephrotic patients with frequent relapses.