A two-phase survey was conducted in the Canton of Vaud among the 1006 registered private practitioners (response rate 98%). The first phase aimed at determining the proportion of these physicians involved in the care of HIV+ persons. The results showed that 43% of the practitioners had been consulted by HIV+ patients. In the second phase, all institutions (hospitals, prisons and IV-drug user rehabilitation and testing centers) and a representative sample of the physicians with HIV+ patients were asked about the transmission category of their HIV+ patients. A mathematical method was used to estimate the true number of known HIV+ individuals by December 1988. Approximately 60% of the HIV+ persons had been seen exclusively by the private practitioners. IV-drug users represented 57% of all HIV+ persons compared to only 27% of the AIDS cases registered in 1988, suggesting that an important change in the transmission categories of AIDS cases is to be expected in the near future. These observations underscore the evolving nature of the HIV epidemic on the one hand, and the crucial role of the private practitioners in the prevention of the HIV infection on the other. This also points to the need for methods specifically designed to monitor HIV and AIDS epidemics respectively.