Retrospective and prospective analyses of heart transplant recipients showed no significant association between acute rejection and the detection of cytomegalovirus (CMV) infection by culture or the polymerase chain reaction (PCR) for viral DNA, neither on grounds of the incidence of both conditions nor in relation to which was diagnosed first in the patient. Semiquantitative PCR of serial blood and endomyocardial biopsy specimens from individual patients revealed different patterns in the development of the viral DNA in the blood and the heart, also clear episodes of CMV infection in CMV antibody-negative recipients of hearts from CMV antibody-negative donors, none of whom went on to develop a CMV-specific antibody response. None of these findings was associated with the development of rejection in the patient. On the other hand, in those patients who did experience rejection, peak levels of CMV DNA in the blood and the heart were usually not reached until 6 weeks or more after transplantation, whereas in those in whom rejection was not detected at all during the period of observation, peak levels of CMV DNA were detected earlier, mainly within the first 6 weeks after transplantation. In several cases, the delayed increase in CMV DNA in those with rejection, albeit not the delay itself, was linked to treatment with steroids. These findings support the view that CMV infection and rejection are independent events, but that the timing of the infection, and whether or not rejection is detected, are indicative of the general status of the immune response in individual patients.