The Authors studied anti-HTLV III antibodies prevalence in jailed people of Perugia jail. Out of 207 healthy subjects, 10.6% were positive to anti-HTLV III. Prevalence in drug abusers (27.5%) was not apparently different from that observed in a control population of addicts who lived in the community. Prevalence in prisoners who were not intravenous drug abusers (2.2%) was higher, when compared with prevalence in general, "not at risk", population. This result can be explained by the exposition to different risk factors, some of which could depend on prisoner status.