The new Communicable Diseases Control Act has come into force in Norway. It makes it compulsory for a physician to warn a third party if it is obvious that a HIV-positive patient, with a high degree of certainty, puts the third party at risk of being infected with HIV. Some philosophers characterize medical confidentiality as an intransigent and absolute obligation, others as a prima facie duty. This article supports the latter view, but the author still argues that strict conditions have to be fulfilled before a physician should consider breaking medical confidentiality: The doctor must try repeatedly to gain the consent or co-operation of the patient involved. Possible negative long-term consequences for the preventive HIV-work support strict medical confidentiality.