We investigated the value of monitoring CMV antigenemia during and after antiviral therapy for CMV disease. During the study period, 10 out of 214 renal transplant recipients were treated for CMV disease, receiving a total of 14 courses of treatment. Antigenemia decreased within 7 days after onset of treatment in eight of nine courses associated with a rapid clinical recovery. In three courses with a slow or absent response, antigenemia levels initially increased. Monitoring antigenemia was helpful in differentiating persisting CMV disease from other opportunistic infections and rejection. Relapses of CMV disease were preceded by rises in antigenemia. Viral isolation became negative within 3 days after initiation of ganciclovir, irrespective of the clinical response. Antigenemia is a marker of the effect of ganciclovir on CMV replication in vivo, and its monitoring may be valuable in the management of patients with severe CMV disease.