We prospectively monitored clinical data and renal function at monthly intervals in 165 patients who had received living-related-donor renal allografts in our institution between January 1981 and December 1991 and had a functioning allograft for 1 year or longer. During a mean follow-up period of 47.2 (range 13-155) months, 32 patients (17.2%) developed late acute rejections, of which 14 (43.7%) were asymptomatic. Amongst the symptomatic late acute rejections, worsening of hypertension was the commonest finding, being present in 11 (61.1%) patients, followed by oliguria in 8 (44.4%) and weight gain in 7 (38.8%) patients. Of these 32 late acute rejections, as many as 28 (87.5%) showed a response to antirejection therapy with high-dose steroids: 5 (15.6%) a complete response and 23 (71.9%) a partial response. The response rate was 100% if it was the first acute rejection (20% complete and 80% partial), 78.6% if it was the second (14.3% complete and 64.3% partial), and no or only a partial response to treatment if it was the third acute rejection episode. On long-term follow-up, patients who had responded to antirejection treatment had a significantly better graft survival as compared with nonresponding patients: 76 and 27%, respectively. Our observations suggest that routine monitoring of the renal function at frequent intervals is essential for early diagnosis and treatment of acute rejections, even during the late posttransplant period. The chances of a response to antirejection therapy are higher during the first episode of late acute transplant rejection as compared with a second or a third late rejection event.