By screening sera obtained from 5015 pregnant women under the care of 9 gynecology/obstetrics departments of hospitals or clinics in Nagasaki City and its surrounding areas, 187 were found to be positive for antibody against adult T-cell leukemia-associated antigen (ATLA). The prevalence of seropositive pregnant women was consistent with that of blood donors in the age group of 16 to 29 years, taken as controls. Essentially all cord blood samples (113/115) of babies born from these seropositive mothers were positive for anti-ATLA of IgG class. These IgG antibodies in the babies diminished rapidly after delivery, and were detectable only in 3 cases at 2, 3, and 5 months of ages out of 38 babies up to 21 months. None of 115 cord bloods so far tested was positive either for anti-ATLA of IgM class or for ATLA-bearing lymphocytes after short-term cultures of the rosette-forming T-lymphocytes. Two of the babies born from 38 seropositive mothers were found seroconverted between the ages of 12 and 19 months. Three out of 20 elder siblings were found seropositive at ages of 3 to 6 years. The seropositive rate in these siblings was significantly higher than that of controls. Moreover, 12 out of 13 mothers traced back from seropositive students were found seropositive. These data indicate strongly that HTLV-I is transmitted from seropositive mothers to their children as a major pathway.