To evaluate the effect of previous AIDS education on AIDS knowledge and sex behaviour among Danish adolescents, a survey was carried out in April-May 1988 amongst 15-16 year-old school children of 9th grade elementary school. Through a cluster-sampling procedure, 45 school classes in nine geographical areas around the country were selected representing various degrees of urbanisation. A questionnaire was handed out and collected by the local school physician during a class session. The 728 pupils responding corresponded to a response rate of 89% of the pupils enlisted and 99% of the pupils present. The results demonstrate that AIDS education has been widely introduced in Danish schools and is well accepted. However, school health services have only rarely been involved. The educational activities seem to have a positive effect on knowledge about HIV-transmission and AIDS, attitudes towards the use of condoms and actual sexual behaviour, although risk-behaviour is still prevalent. The respondents express positive attitudes towards more AIDS education--especially among those with very little or no previous educational experiences.