Adult T-cell leukemia/lymphoma (ATL), a malignancy of mature CD4-positive lymphocytes, has been etiologically linked to the human retrovirus HTLV-I. Although a long latent period is suggested from migrant studies, little prospective information on the risk of developing ATL among persons with HTLV-I infection is available. We present here a model for ATL risk based upon age- and sex-specific HTLV-I seroprevalence data from a cross-sectional survey of 13,000 Jamaicans and ATL incidence data from a 21/2-year case-control study. By examining the age-specific incidence of ATL relative to both adult and childhood-acquired seropositivity versus childhood-acquired seropositivity alone, we provide evidence in support of the hypothesis that childhood infection with HTLV-I is important to the development of ATL. Using this model, the cumulative lifetime risk of ATL for those infected before age 20 is estimated to be 4.0% for males and 4.2% for females. Under this hypothesis, HTLV-I-associated diseases with shorter latent periods, such as tropical spastic paraparesis, should have a higher incidence in adult females than in adult males.