We evaluated 95 HIV seropositive drug-addicts during a follow-up period of one year. The patients were classified and reclassified at each visit, according to the criteria proposed by the Centers for Disease Control (CDC) in 1986. At the first visit the patients were classified as follows: 70 in IIa-IIIa, 23 in IIb-IIIb and 2 in group IV. a and b indicate the absence or the presence of immunological and/or hematological alterations. 1 out of 70 IIa-IIIa patients versus 8 out of 23 IIb-IIIb patients developed AIDS in one year (p less than 0.001). Only one patient belonging to IIa-IIIa groups at the first visit progressed to AIDS in one year. This patient was reclassified in subgroup b after six months. No differences were noted among patients classified in IIb (5 patients) and IIIb (4 patients) who progressed to AIDS. Taken together these data indicate that the belonging to b subgroup is a risk factor for developing AIDS and the passage through a b subgroup (II or III) is a necessary step before the appearance of clinical manifestations of AIDS.