The presence of anti-HIV (human immunodeficiency virus) antibodies was determined in 325 subjects. The samples were taken from 138 convicts from Carabanchel and Ocaña I jail, 162 parenteral drug addicts (PDA) from a center for rehabilitation of drug addicts (CR), and from 14 family members and 11 stable couples of these PDAs. There was a global seroprevalence of 61.23%, 56.9% in Carabanchel, 68.7% in Ocaña I and 68.5% in the CR. When the results from the jailed convicts were analyzed according to risk factors, it was discovered that drug addiction was the most relevant since there was only a 29.4% of seropositivity in non PDAs with a statistically significant difference (p less than 0.01). This value was also higher than the seroprevalence found in the CR with a statistically significant difference (p less than 0.01). None of the family members of the CR were HIV-positive. Two stable couples were positive in this sexual contact group. This article makes clear the important role of drug addiction in the dispersion of HIV infection in our midst.