Cytomegalovirus (CMV) disease is a serious complication after bone marrow transplantation (BMT). Recently, semi-quantitative polymerase chain reaction (SQ-PCR) analysis for CMV-DNA has been developed. We compared the time course of SQ-PCR and that of antigenaemia assay after BMT. Our findings suggested that SQ-PCR is more sensitive than antigenaemia assay in the detection of CMV infection and that quantitation of CMV-DNA could discriminate patients with positive PCR but without the risk for CMV disease. We considered that SQ-PCR could be a useful guide for starting pre-emptive therapy against CMV disease.