Cyclosporin (CYA) is now recognized as an effective immunosuppressant to lead to a marked improvement in graft survival in organ transplant recipients. Although the incidence of infection in the CYA group has been decreased compared with that in the azathioprine group, infectious diseases in 400 kidney transplant recipients treated with CYA were noted in our single center. Treatment strategy for infectious diseases: Antibiotics and/or gamma-Globulin were administered to all recipients with bacterial infections. Aciclovir was added in recipients with herpes simplex virus (HSV) infection or varicella zoster virus (VZV) infection. Human interferon-beta (HuIFN-beta) was used in recipients who had life-threatening viral infection, especially cytomegalovirus (CMV) pneumonitis. Glycyrrhizin was used for acute hemorrhagic cystitis and nephropathy due to adenovirus (AV). Trimethoprim sulfamethoxazole and/or pentamidine were added in recipients complicated with Pneumocystis carinii (Pc) pneumonitis or in order to prevent Pc pneumonitis. Infectious diseases: One hundred and six recipients had infectious diseases 129 times in this series, seventy-six percent of all infections occurred during the first 4 months after the transplantation. Urinary tract infection (UTI), herpes zoster and pulmonary infection were the most common infectious diseases, occurring in 28.7%, 24.0% and 23.2%, respectively. Septicemia or bacteremia developed in 9 recipients, secondary to UTI in 8 and to surgical wound infection in one. Sixty-one symptomatic viral infections occurred in 57 recipients. A total of 5 recipients (1.3%) died of interstitial pneumonitis. Infectious organisms: Viral and bacterial infections were most common, occurring in 47.3% and 41.9%, respectively. Viral species detected in these recipients with the frequency were HSV 14 times, CMV 9 times, VZV 31 times and AV 7 times. 1) The incidence of viral infections in kidney transplant recipients treated with CYA is relatively high compared to bacterial infections. 2) HuIFN-beta therapy is effective in the treatment of serious opportunistic herpes virus infections, especially CMV pneumonitis. 3) Glycyrrhizin therapy is effective in the treatment of acute hemorrhagic cystitis and nephropathy due to AV and hepatic dysfunction. 4) Aerosolised pentamidine therapy is very useful for prophylaxis of Pc pneumonitis.