To determine the prevalence of HTLV I-II among highly sexually active adults, a seroprevalence study was conducted in Italy between March 1992 and March 1993 among 1506 patients with a newly diagnosed STD. Individuals were tested with an enzyme immunoassay that employs a double viral lysate from HTLV-I and HTLV-II infected cell lines; confirmation and discrimination between HTLV-I and II were performed by a dot-blot immunoassay. One patient (0.07%) was confirmed HTLV-I positive and one (0.07%) HTLV-II positive. The HTLV-I positive was a Romanian woman, and the other was an intravenous drug user. The 0.13% HTLV prevalence observed in this study is intermediate between the 0.034% found among Italian blood donors and the 3.5-6% found among Italian intravenous drug users. These data suggest that there is not a high prevalence of these viruses among Italians with STDs.