Over 80% of cases of Acquired Immune Deficiency Syndrome (AIDS) in England and Wales have occurred in homosexual men. Changes in sexual behaviour in this group may have a substantial influence on the incidence of Human Immunodeficiency Virus (HIV) infection and will therefore be crucial in determining future cases of AIDS. This paper critically weighs the indirect and direct evidence for changes in behaviour in homosexual men since the advent of the AIDS epidemic. The paper reports on falling incidence of gonorrhoea, hepatitis B and syphilis in homosexual men, the changes being most marked from 1985 onwards. Data on temporal trends in HIV prevalence and incidence in homosexual men are reviewed. These suggest that the maximum incidence of HIV infection occurred in 1982-84 and may have fallen since then. Evidence for a concomitant change in sexual behaviour is reported from several sources. This points towards a recent change in sexual behaviour characterized by reduction in the numbers of partners and adoption of safer sexual practices. In some places change may have occurred as early as 1983. A change became apparent generally in 1985 and this appears to have been sustained in 1986-87. Nevertheless, a substantial proportion of homosexual men studied continue to practice high risk sexual practices, such as anal intercourse, including relationships with casual partners.