We developed a polymerase chain reaction (PCR) method which has high sensitivity and simple technique in order to investigate the presence or absence of human T lymphotropic virus type I (HTLV‐I) provirus in cord blood mononuclear cells of neonates born to HTLV‐I carrier mothers. Out of 40, three subjects were found to contain the HTLV‐I provirus genome. These three subjects remained HTLV‐I sequence‐positive in follow‐up study. On the other hand, when examined by a conventional technique for detection of HTLV‐I‐associated antigen on peripheral mononuclear cells, all 40 neonates were HTLV‐I‐associated antigen‐negative. These results suggest that PCR is more sensitive than the conventional antigen detection method and is useful in early detection of HTLV‐I infection in neonates born to HTLV‐I carriers.