Two hundred twenty-eight plasma specimens collected over a 4-year period from 185 children aged 4 days to 10 years at risk for human immunodeficiency virus (HIV) infection were tested for antibody to human T-cell lymphotropic virus type II (HTLV-II) to determine perinatal transmission of this agent. One hundred three of the specimens were from 68 children whose mothers were intravenous drug users (IVDU). None of the children in the study were breast fed. No HTLV antibody was detected in any of the samples from children of non-IVDU mothers. However, HTLV-II antibody was found in samples from 15 of 38 infants < 6 months old born to IVDU mothers. Later specimens taken at 8-16 months from six of these children were negative, suggesting passive maternal antibody, not infant infection. The 62 samples from 30 children 6 months to 7 years old from IVDU mothers were negative for HTLV antibody. The lack of HTLV antibody in the older children indicates that no vertical transmission had occurred. Based on the prevalence rate of maternal HTLV-II antibody found in the children < 6 months old, an HTLV-II infection rate of approximately 36% was projected for the IVDU mothers in the study.