Ten years after the arrival of the Human Immunodeficiency Virus (HIV) in the developed countries a total of 45000 AIDS cases have been reported, 75% of them in the USA. Perhaps 1–2 million persons have been infected. Although two thirds of all AIDS cases in developed countries have been in homosexual men, there is evidence that the spread of AIDS in homosexual men has slowed, so that AIDS in drug-users and heterosexually transmitted AIDS are becoming increasingly important. The principal routes for the movement of HIV into the general heterosexual population in developed countries are from intravenous drug-users and from Third World countries where HIV is heterosexually established. At present there is no evidence of widespread heterosexual transmission within any of the developed countries; but, in the absence of effective treatment or vaccines, there is a clear potential for a slow build-up of heterosexual seropositives.