Between June 1989 and May 1991, 29,020 subjects were tested for anti-human immunodeficiency virus (HIV) antibodies; 1079 were seropositive. A big discrepancy existed between the high seroprevalence in the group 0-3 yr of age, 11.22%, and that of 0.43% in the adult population. Moreover, only a small percentage of the pediatric cases (4%) resulted from maternal infection. These observations indicate viral spread by horizontal transmission. Blood, blood products, and reusable unsterilized needles and syringes were probably the main route by which HIV spread in the infant population in medical institutions. The initial source of HIV infection in Romania remains to be determined.