We developed a new method to detect human T cell leukemia virus type I (HTLV-I) provirus by amplifying the pX region of the HTLV-I genome by means of the polymerase chain reaction (PCR) method. This new method was used to examine babies delivered by HTLV-I carrier women for HTLV-I vertical transmission. It was found that intra-uterine infection with HTLV-I can occur, though rarely, and some babies carry HTLV-I provirus despite their negative response to HTLV-I antibody, and, noticeably, some of the babies found negative by the conventional antigen detection method may be positive when tested by the PCR method. It was also confirmed with the PCR method that the incidence of vertical transmission of HTLV-I can be reduced to about 1/5 by replacing breast feeding with bottle feeding and frozen mother's milk feeding. It was also found that vertical transmission of HTLV-I can occur at high incidence rates even when the duration of breast feeding is 3 months or less.