CMV disease often recurs after initially successful antiviral therapy. We retrospectively determined in a group of 36 organ transplant patients whether clinical, virological, or immunological parameters during or shortly after cessation of antiviral therapy can identify those at high risk of relapse. Eleven of 36 patients had recurrent CMV disease after ganciclovir therapy. Neither donor or recipient CMV serostatus, type of baseline immunosuppression, antirejection treatment, indication for antiviral treatment, nor presence of CMV in the blood during or after therapy (as detected by antigenemia, viremia, or a positive polymerase-chain-reaction signal) were helpful in identification of patients with subsequent relapse. However, quantitative monitoring of antigenemia fascilitated early diagnosis of relapse since 10 of 11 patients with > or = 10 antigen-positive cells per 50,000 PMNs relapsed (99.1%, 95% CI 58.7-99.8). IgM and IgG responses against CMV during primary infection were comparable in relapsing and nonrelapsing patients. During secondary infection relapse occurred only in the 4 patients with the lowest IgG responses. The number of activated CD8bright lymphocytes in the peripheral blood as determined by flow cytometry at the end of antiviral therapy was a strong risk factor for the subsequent clinical course: 6 of 7 patients (85.7%, 95% CI 42.1-99.6%) with < 100 x 10(3) HLADR+CD8bright cells/ml blood relapsed, while 8 of 8 (100%, 95% CI 63-100) with activated CD8bright cells above that level remained asymptomatic (P < .025). These data show that patients with a high risk of relapse of CMV disease can be identified at the end of antiviral therapy.