Forty patients with serum antibody against herpes simplex virus (HSV) were enrolled in a randomized, placebo-controlled, double-blind investigation of acyclovir given orally in a low dosage as prophylaxis against recurrent HSV infection after renal transplantation. During 30 postoperative days of medication, 14 of 21 placebo-treated and one of 19 acyclovir-treated patient(s) developed reactivation of HSV infection. Eleven of the former, but not the latter, had herpetic lesions. The protection against active infection with HSV during the period of prophylaxis with acyclovir is statistically highly significant. From 30 to 90 days after transplantation when no antiviral medicine was given, 60% (3/5) of the remaining placebo recipients and 44% (7/16) of the acyclovir patients developed active HSV infections. Herpetic lesions occurred in two of three and two of seven of infected people in the respective groups. No adverse effects of the drug were observed. The results show that HSV infections in immunosuppressed renal allograft recipients can be safely prevented, deferred, and ameliorated by an initial period of prophylaxis with a low dose of oral acyclovir.