Since the beginning of the acquired immunodeficiency syndrome (AIDS) epidemic, the systems for epidemiological surveillance have monitored the dynamics of spread of the disease, thus permitting to identify the possible risk factors and the evaluation of the preventive measures that have been carried out. In the present article the data collected in the AIDS registry for Catalonia are reported. In contrast with the overall figures for the rest of Spain, in Catalonia the most numerous group of patients was represented by homosexual or bisexual males until the second half of 1986; since then, parenteral drug abusers became the major group of risk as in other countries from Southern Europe. In addition, a consistent increase in the number of involved females and children and a small increase in those cases where the only possible known route of transmission was heterosexual contact was observed.