Cyclosporin-A-treated renal allograft recipients have demonstrated an improved graft survival rate, when compared to that of patients treated with conventional azathioprine and steroid therapy. Cyclosporin-A has been used for immunosuppressive therapy after renal transplantation at the National Taiwan University Hospital since November 1985. Since then, the one-year graft survival rate has been 78%, and the patient survival rate is 91%. At our service, acute rejection is confirmed mainly by an increase in the serum creatinine level of 0.5 mg% per day and a subsequent return of kidney function to normal after pulse steroid therapy. Twenty patients receiving cyclosporin-A and suffering from acute rejection episodes were chosen for comparison with 18 patients receiving conventional azathioprine and steroid therapy. Compared to conventional therapy, the classical systemic manifestations of rejection, such as malaise, lethargy, apathy, general weakness, vague discomfort, increase in body weight, swelling of graft with tenderness, were all more mild and less frequent in the cyclosporin-A-treated group. Episodes of rejection appeared earlier and the duration of rejection was shorter than in those of the conventional group. The urinary sodium concentration and the ratio of urine urea nitrogen to blood urea nitrogen were reliable references during the acute rejection episode in the conventional group, but it cannot be used as indices in the cyclosporine group. These findings can help us understand the changes which occurred in acute rejection in patients who receive renal transplantation during the cyclosporine era.