The relationship between cytomegalovirus (CMV) infection and cardiac allograft rejection is controversial, some authors reporting a significant association, others not, on the basis of the results of conventional virological diagnosis by culture or serology. This problem was reinvestigated in 88 patients using a semi-quantitative nest polymerase chain reaction (PCR) procedure for detecting CMV DNA in endomyocardial biopsy specimens. Significantly more positive biopsies were obtained from patients with moderate (grade 2; P = 0.02) or severe (grade 3a-4; P = 0.03) rejection than with no or mild (grade 0-1b) rejection, whereas there was no significant association between rejection and CMV as diagnosed by virus isolation from urine, throat or blood, or by the detection of CMV-IgM. PCR-positive biopsies originated most frequently from CMV-antibody positive recipients (R+) of hearts from seropositive donors (D+), in association with moderate or severe rejection rather than with mild or no rejection The detection of CMV in the heart thus seemed to be related more to R+D+ serological status than to severity of rejection, that is, to circumstances that favoured co-infection with strains of CMV from both donor and recipient. Studies on sequential biopsy specimens from selected patients also provided evidence that CMV infection and rejection were not always related events. The PCR was able to differentiate latent from active CMV infection; moreover, some seronegative individuals gave repeatedly positive biopsies, thereby supporting the work of others that some patients undergo CMV infection without mounting a detectable antibody response.(ABSTRACT TRUNCATED AT 250 WORDS)