To examine the distribution of AIDS-defining illnesses among Danish AIDS patients, data on 687 AIDS patients diagnosed in the period from 1980 to 1990 (93% of all reported cases in the period) were collected. The most frequent AIDS-defining illness was Pneumocystis carinii pneumonia followed by candida oesophagitis and Kaposis sarcoma. The proportion of homo/bisexual men presenting with Kaposis sarcoma as the initial AIDS-defining illness declined over time. Patients with extrapulmonary tuberculosis had higher CD4 cell counts than patients presenting with other illnesses. Cytomegalovirus chorioretinitis and atypical mycobacteriosis were seen more frequently after the time of the AIDS diagnosis, and a low CD4 cell count at time of the AIDS diagnosis was a significant predictor for the development of these opportunistic infections during follow-up. Danish AIDS patients present with a wide spectrum of HIV-related illnesses, reflecting their exposure to opportunistic microorganisms and the degree of immune deficiency. The pattern of HIV-related illnesses is changing over time, and therefore continuous surveillance is needed to optimize therapeutic and prophylactic regimens.