The seroprevalence of human T-cell leukemia virus type I (HTLV-I) antibody in 6701 healthy females and 226 women with gynecologic malignancies, all living in an adult T-cell leukemia-endemic area in southwestern Japan, was investigated to determine whether HTLV-I infection was a risk factor influencing oncogenesis and prognosis. The seroprevalences in cervical carcinoma patients younger than 59 years and in vaginal carcinoma patients of all ages were significantly higher than in age-matched healthy controls. The ratios of observed to expected HTLV-I seroprevalence in patients younger than 59 with cervical carcinoma and in vaginal carcinoma patients were 2.92 and 7.36, respectively. Among the patients with cervical carcinoma or vaginal carcinoma, the tumor recurrence rate in HTLV-I carriers was significantly higher than that in HTLV-I seronegative patients. Our results suggest that HTLV-I infection may be oncogenic and may affect the prognosis in some patients with cervical or vaginal carcinoma.