Human T-cell lymphotropic virus type I (HTLV-I) was associated with carcinoma of the cervix in Japan in a recent study that compared hospital cases with healthy population-based controls. To test this relationship in women more alike for cervical neoplasia risk factors (including sexual behavior and human papilloma virus; HPV), we enrolled consecutive patients from a colposcopy clinic in Kingston, Jamaica (an HTLV-I endemic area). Patients underwent Pap smear, colposcopy, biopsy and cervical swab for detection of HPV by polymerase chain reaction. Cases were defined as women with CIN-3 or invasive cancer (CIN-3/CA). Controls included all patients with either CIN-I or koilocytotic atypia, atypical squamous cells of undetermined significance or benign cervical pathology (all but one had at least inflammatory changes). Patients with CIN-2 were excluded to minimize risk of case-control misclassification. Cases were much more likely to be HTLV-I seropositive than controls. Although mean age differed significantly between cases (mean age = 39 years) and controls (mean age = 33 years), control for age did not explain the relation of CIN-3/CA with HTLV-I. Among HPV DNA positive subjects the age-adjusted association was not diminished but lost statistical significance. HTLV-I seroprevalence may be independently associated with progression to severe neoplasia of the cervix.