The transmissibility of human T lymphotropic virus (HTLV) type II from mother to child was investigated. Of 236 women enrolled during pregnancy in a study of mother-to-child transmission of human immunodeficiency virus in 1986-1988, 21 (8.9%) were seropositive for HTLV-I/II. All 21 mothers were infected with HTLV-II by synthetic peptide testing and polymerase chain reaction (PCR). HTLV-II-infected women were older (median age, 34 vs. 28 years), more likely to be black (70% vs. 38%), and more likely to report past or current intravenous drug use (85% vs. 56%) than HTLV-II-uninfected women. Of 20 non-breast-fed infants born to 19 of these HTLV-II-infected women, none had detectable HTLV-II by PCR done on peripheral blood mononuclear cells obtained at birth to 36 months of age. Serologic testing of these infants revealed gradual disappearance of HTLV-I/II antibody. While this study does not rule out the possibility of perinatal HTLV-II transmission, the data suggest that it occurs rarely in the absence of breast-feeding.