Objectives:To study trends in AIDS incidence in Europe by age and year of birth.Design:Age-period-cohort analyses were adopted to distinguish the different time factors of calendar year, age and year of birth.Methods:Non-aggregate AIDS incidence data from 12 European countries (1978–1994) were adjusted for reporting delay and expressed per unit of population (per 100 000 persons or 100 000 person-years). Age-specific incidence patterns (absolute level and rate of increase) were compared between 5-year birth cohorts for homo-/bisexual men, injecting drug users (IDU) and heterosexual contact cases.Results:Mean age at diagnosis increased strongly amongst IDU, but less so among homo-/bisexual men and heterosexual contact cases. Of a total 110 646 reported cases (116 311 after adjustment for reporting delay), 87 167 (78.8%) were among people born in 1950–1974 [91 951 (79.1%) after adjustment for reporting delay]. The relative impact on specific birth cohorts differed strongly by exposure group. Incidences at age ranges of 20–24 and 25–29 years among cohorts born in 1965 and after were about the same level (homo-/bisexual men, IDU) or higher (heterosexual contact cases) than older birth cohorts when these were in the same age range; rates of increase were less among homo-/bisexual men and IDU, but higher among heterosexuals. There were large differences between counties.Conclusions:Overall, AIDS incidence among cohorts born in 1965 and after is about the same level (homo-/bisexual men, IDU) or higher (heterosexual contact cases) than older cohorts when these were in the same age range. Rates of increase of AIDS incidence curves suggest reduced HIV transmission amongst the most recent cohorts of homo-/bisexual men and IDU, but among young heterosexuals the epidemic is still expanding.