High dose oral acyclovir has been reported to be effective in preventing both cytomegalovirus (CMV) infection and disease in renal transplant recipients. We conducted a case-controlled study in which 42 cadaveric kidney transplant recipients were prophylactically treated with high dose oral acyclovir for 3 months and compared to historical controls matched for donor/recipient CMV serological status, age, sex, and immunosuppressive therapy. Before transplantation, study group patients received acyclovir intravenously (500 mg/m2 over 1 hour) which was subsequently given orally (basal dose--800 mg four times daily) from day 2 post-transplantation according to renal function. All patients received 14-day induction immunosuppressive therapy with either a polyclonal or a monoclonal antibody together with low dose steroids and azathioprine, cyclosporin being introduced at day 10 post-transplantation. Blood viral cultures as well as CMV antibody titers were performed in study group and control patients in the same laboratory, before transplantation, weekly until 3 months and then monthly until 6 months. CMV infection was defined as a positive blood or bronchoalveolar lavage viral culture or presence of CMV IgM or CMV IgG in a previously seronegative patient. Diagnosis of CMV disease also required the presence of at least one concomitant febrile illness, with or without other clinical symptoms, not attributable to another pathogen. All patients were followed for 3 months. Incidence of both CMV infection and disease was compared in the two groups using the log-rank test. With regard to CMV infection, we found significantly less CMV infection in CMV seropositive patients (regardless of donor CMV serological status) in the study group compared to historical controls (P = 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)