Significant morbidity and mortality are associated with primary cytomegalovirus infections in renal-transplant recipients. In the hope that immunity to cytomegalovirus could safely be established before transplantation, we vaccinated 12 seronegative renal-transplant candidates with the Towne 125 strain of live human cytomegalovirus. Before transplantation, there were no significant reactions except for erythema and induration at the site of inoculation. All vaccinees seroconverted, and the three patients tested acquired a cytomegalovirus-specific cellular immune response. Ten vaccinees underwent transplantation: Nine have completed at least 3 months of follow-up, and eight retain functioning allografts up to 1 year later. Although cytomegalovirus was isolated from six patients after transplantation, the restriction endonuclease patterns of the viral DNA of four of these isolates differed significantly from those of the vaccine strain. Therefore, it appears that the vaccine strain did not become latent in the host, at least in a form that could be reactivated.