Objective:To assess the repercussion of late diagnosis of HIV infection on AIDS incidence in the era of highly active antiretroviral therapy.Design:Analysis of AIDS surveillance data.Setting:Spain.Patients:AIDS cases reported from 1994 though 2000.Main outcome measures:Late testers were defined as persons who had a first positive HIV test in the month of or immediately preceding AIDS diagnosis. Their incidence trend was compared against that for the remaining cases, and the influence of demographic factors evaluated.Results:Of 30 778 AIDS cases, 8499 (28%) were late testers, and of these, 1061 (12%) died within 3 months of diagnosis of HIV infection. From 1995 to 2000, AIDS diagnoses declined by 36% among late testers versus 67% for the remainder (P< 0.001). The percentage of late testers increased from 24% in 1994–1996 to 35% in 1998–2000 (P< 0.001). Among the 7825 AIDS cases diagnosed in 1998–2000, late testing was independently associated (P< 0.01) with male sex, age over 44 years, residence in provinces with a lower AIDS incidence, foreign origin, and no record of injecting drug use or prison stay.Conclusions:A growing proportion of AIDS cases involves late diagnosis of HIV infection. Persons who are unaware of their HIV infection cannot benefit from antiretroviral therapy and, hence, early diagnosis would strengthen the impact of such therapy and so reduce AIDS incidence.