Primary prophylaxis by a change of behaviour in the population continues to be the only effective possibility to prevent a further spread of the aids epidemic. The most important presupposition for this is the precise knowledge of the ways of transmission and the corresponding protective measures. Informations referring to this must turn up repeatedly and above all free of contradictions at all levels of the preventive campaign. The desired changes of behaviour suppose also a climate of confidence and solidarity. The physician's task of prevention is part of the comprehensive strategy of HIV-prevention in Switzerland (effect in width and depth on population- and target groups and on the individual level). It is naturally the physician's domain to look after and counsel individually. Most variable possibilities of beginning a conversation about Aids prevention offer themselves to him. He should put the indication for this as generous as possible and deal extensively with a possible risky behaviour which might not be conscious to both the patient and the physician. The HIV antibody test is indicated in certain situations, however the pre- and posttest counselling is in any case much more decisive than the result of the test. The preventive measures recommended up to now for the most important ways of transmission (sexual contact, intravenous drug use, perinatal transmission, blood/blood products) are still valid without modification.