91 renal transplant candidates were randomised to receive Towne strain cytomegalovirus (CMV) vaccine or placebo at least 8 weeks before transplantation. The vaccine was well tolerated and there was no vaccine virus excretion. Serological and cellular immune responses developed in most vaccines but were lower in the transplant patients than in healthy volunteers and some of the seronegative patients failed to mount responses. CMV infection occurred in most of the seronegative vaccine-treated or placebo-treated patients who received kidneys from seropositive donors, but the illnesses were less severe in the vaccines than those in similarly exposed placebo-treated patients. Vaccine-treated patients who received kidneys from seronegative donors did not excrete virus, and therefore the vaccine virus was not reactivated from a putative latent state despite immunosuppression at the time of transplantation.