In a prospective study, the cytomegalovirus (CMV) pp65 antigenemia assay was compared with detection of CMVby blood culture in 67 consecutive CMV-seropositivepatients undergoing autologous marrow or peripheral blood stem cell transplantation. Antigenemia occurred in 26 patients (38.8%) a median of 33 days (range, 12–74) after transplant. Viremia was detected in 5 patients (7.5%) a median of 30 days (range, 12–74) after transplant and was treated with ganciclovir in 3. Antigenemiaoccurred sporadically at lowlevels (S positive cells/slidedeveloped fatal CMVpneumonia 8 days later. Neither patient had CMV viremia before onset of pneumonia. Thus, quantitative CMV pp6S antigenemia may be useful in guiding antiviral treatment in seropositive autograft recipients.