Cancer incidence is enhanced in transplant recipients. Decreased DNA repair ability is associated with increased cancer incidence. Transplanted patients with cancer were found to have reduced DNA repair. We hypothesized that immunosuppressive therapy may impair DNA repair and thus contribute to the increased cancer incidence in transplanted patients. The objectives of this study were (1) to investigate the effect of two immunosuppressive treatment protocols on DNA repair in kidney transplant recipients; (2) to evaluate the cancer incidence in these patients; and (3) to study the in vitro effect of cyclosporin A (CsA), azathioprine, and prednisolone - Separately and in various combinations - On DNA repair. Three groups were studied: (1) a control group; (2) patients treated with azathioprine and prednisone (double-therapy group); and (3) patients treated with CsA, azathioprine, and prednisone (triple-therapy group). The two patient groups did not differ in age, gender, time on dialysis before transplantation, or kidney function or in the number of acute rejections. However, the interval from transplantation to the DNA repair study was shorter in the triple-therapy group (P < .01). DNA repair was induced in peripheral blood mononuclear cells (PBMCs) by ultraviolet irradiation and expressed as tritiated thymidine uptake by these cells. DNA repair in the tripletherapy group was 679 ± 64 cpm/106 cells, significantly less than that in the control group (1049 ± 69 cpm/106 cells, P < .02). In the double-therapy group, DNA repair was similar to that in the control group. The follow-up period was shorter in the triple-therapy group (116 ± 19 months vs 174 ± 29 months, P< .01). Five tumors developed in the triple-therapy group, but only one developed in the double therapy group (P = .05). The in vitro study showed a dose-dependent reduction in PBMC DNA repair by CsA. Azathioprine and prednisolone reduced DNA repair slightly, but CsA reduced DNA repair significantly more than either one or a combination of them. In summary, triple therapy was associated with impaired PBMC DNA repair and increased cancer incidence. CsA was responsible in large part for the reduction in DNA repair ability found in the in vitro and in vivo studies. This may have partly contributed to the enhanced cancer incidence in the kidney transplant recipients. (J Lab Clin Med 2001;137:14-20).