Objectives: To present the current epidemiology of prevalent diagnosed HIV infections in England, Wales, and Northern Ireland (E, W, & NI) and describe trends over time. Methods: Descriptive analyses of the annual national Survey of Prevalent HIV Infections Diagnosed (SOPHID) for the period 1997 to 2003. Results: In 2003, 34 251 adults (15 years of age or over) were seen for HIV related care in E, W, & NI, representing a 17% increase in the prevalence of diagnosed HIV infections compared with 2002 and a 132% increase compared with 1997. Between 1997 and 2003, as a proportion of total prevalent cases, adults who acquired their infection through heterosexual sex increased from 26% to 49%; black African adults increased from 15% to 35% and diagnosed adults resident in London fell from 62% to 55% of the total. The male to female ratio declined from 5:1 to 2:1. The proportion of adults receiving combination antiretroviral therapy increased from 53% in 1998 to 64% in 2003. Conclusion: There has been a large increase in the number of adults with diagnosed HIV infection seen for care in E, W, & NI since 1997. Changes in the epidemiology of prevalent diagnosed HIV were seen by sex, route of infection, ethnicity, level of antiretroviral therapy, and areas of residence and treatment. In 2003, for the first time, prevalent diagnosed infections acquired through heterosexual sex overtook those acquired through sex between men. These increases have serious implications for the planning and financing of HIV services in the United Kingdom.