The surveillance of the epidemic of infection with the human immunodeficiency virus (HIV) in the United Kingdom is described. Up to 30 September 1987, 1067 cases meeting the World Health Organization/Centers for Disease Control (U.S.A.) definition of the acquired immune deficiency syndrome (AIDS) were reported in the U.K. Of these, 49 were in persons who came to the U.K. for diagnosis and treatment and 1018 were in U.K. residents. There were 7561 laboratory reports of HIV antibody-positive tests in the same period. The main epidemic continued among homosexuals and bisexuals, principally in the four Thames National Health Service (NHS) regions. Laboratory reports of new HIV antibody-positive tests also increased, but the rise was more erratic. For example, a sharp rise in the last quarter of 1986 and the first quarter of 1987 was probably due to a substantial rise in testing which followed the Government publicity campaigns and may not, therefore, necessarily indicate an increasing incidence. AIDS in haemophiliacs increased but new HIV antibody-positive reports declined after the introduction of control measures in 1985. The epidemic increased in intravenous (IV) drug abusers. Although only 32 cases of AIDS were reported, the increase in the number of laboratory reports of HIV antibody-positive tests, particularly in Scotland, suggests that reports of AIDS will increase in this group in the next few years. Heterosexual spread of infection took place but most infections were acquired in sub-Saharan Africa. Nevertheless, five cases of AIDS and at least 16 HIV antibody-positive tests were in persons infected in the U.K. in whom neither they nor their sexual partners had recognised risk factors. Furthermore, it is possible that some of the infections in IV drug abusers were heterosexually acquired. The epidemic of AIDS in the U.K. continues to increase exponentially, new reports of AIDS cases doubling every 10 to 11 months. If this continues there are likely to be about 1700 new cases reported in the 15 months following September 1987, leading to an estimated cumulative total of over 2500 reported cases in U.K. residents by December 1988.