Eighteen laboratories, which together provide primary HIV antibody testing for 43% of the population in England, collaborated in a study to record epidemiological information for all individuals voluntarily tested by them over a five year period. From the 184,113 individuals who had a first test during the study period, it is estimated that 1 in 12 adults in London, and 1 in 50 outside London have been voluntarily tested for HIV since testing became widely available in 1985. The majority of those tested were individuals whose perceived risk was heterosexual exposure. Infection in this group was concentrated in individuals whose partner had an identified risk and in those who had lived in or visited Africa. The rise in antibody prevalence observed in the latter group during 1990/91 may have been partly due to infection recently acquired in the UK. Antibody prevalence in heterosexuals without a high risk partner or a history of exposure abroad also rose during the study period, suggesting a recent increase in transmission through casual heterosexual exposure in the UK. The study also provided strong evidence of continuing high risk behaviour among homosexual men, particularly in the younger age groups. Homosexuals aged under 30 years and living in London had the greatest risk of acquiring HIV infection since 1988.