Thirty-two pediatric renal transplant patients receiving cyclosporin and 34 receiving azathioprine treatment (historical controls) were investigated for the occurrence of rejection episodes; their clinical symptoms and findings, time of onset, influence of donorship, relation to cyclosporin blood levels and graft function outcome were also studied. In the cyclosporin group, four grafts were lost in the 2nd year, while in the azathioprine group five grafts were lost within the first 5 weeks after transplantation due to acute irreversible rejection. Clinical signs of rejection episodes under cyclosporin were mild and usually presented a silent increase of serum creatinine. First rejection episodes occurred later in patients treated with cyclosporin than in azathioprine-treated patients (50% probability after 7 weeks as opposed to 2 weeks). The percentage of patients receiving cyclosporin who had experienced no rejection episodes was 18.8% as opposed to 11.8% of patients receiving azathioprine. The lowest incidence of rejection episodes was observed in patients with living related grafts receiving cyclosporin treatment, 75% of whom were free of rejection episodes after 2 years. Cyclosporin blood levels below 400 ng/ml were observed in 74% of rejection episodes. Biopsies were often used to differentiate between cyclosporin nephrotoxicity and rejection when the cyclosporin levels were above 400 ng/ml. Both treatment groups exhibited a parallel decline in graft function, which correlated with the number of rejection episodes.