During CMV viremia, the CMV specific lower matrix protein CMV pp65 can be detected in the nucleus of polymorphonuclear cells. A relationship has been found between the number of CMV pp65 positive cells, the clinical course and the effect of antiviral treatment on CMV disease. From 1990, heart recipients (triple drug therapy) were screened for CMV pp65 (antigenemia, according to the method described by The et al.), anti-CMV-IgM and -IgG. Tests were repeated at least every 4 weeks. Group 1 consisted of 23 patients who had been transplanted at least one year before the introduction of CMV testing as described. Between 1990 and 1992 26 patients were followed up during the first year after transplantation and represent group 2. In group 1, 1184 antigenemia assays were performed and 13 tested positive. In group 2 (1195 tests, 261 positive results), 20 out of the 26 recipients tested positive for CMV pp65. Without preceding evidence of a positive CMV pp65, no rise of IgM or IgG antibodies was observed. The time until the first antigenemia (time from detection until a subsequent test remains negative); 13 were found in group 1, 84 in group 2. In group 2, 46 episodes of antigenemia (mean duration 24.5 +/- 27.1 days) consisted of more than 1 consecutive positive result of the antigenemia assay (4.8 +/- 4.1). During these episodes the white blood cell count was 3460 +/- 1790/mm3. After the episodes, the mean leucocyte count was 6320 +/- 1870/mm3. The detection of CMV antigenemia indicated the initiation of antiviral treatment (hyperimmune globulin and ganciclovir). Therapy was stopped again when the antigenemia assay tested negative again. Antigenemia disappeared in all patients after initiation of antiviral treatment, CMV disease was not observed. CMV antigenemia mainly cumulates within the first year after heart transplantation. Antigenemia directed antiviral therapy does not prevent infection or repeated antigenemia but prevents CMV disease after heart transplantation.