The role of the Tuscany population-based Cancer Registry (TCR) in the assessment of cancer incidence in AIDS patients, and the completeness of cancer reporting to the Italian AIDS surveillance system (RAIDS) was evaluated through a linkage between the TCR and the RAIDS in the period 1985-90. In the Province of Florence, the incidence of Kaposi's sarcoma in AIDS cases was underestimated by 24% (95% CI; 9.8%-47%; 6/25 cases) by RAIDS in comparison with the TCR. Of kaposi's sarcomas unknown to RAIDS, 2 were incident at the time of AIDS diagnosis ("truly" unreported cases) and 4 were late manifestations of AIDS. Moreover, 1 non-Hodgkin lymphoma unknown to RAIDS and 10 other malignancies (4 lung cancers) were identified through the TCR. In AIDS patients, the incidence of lung cancer was 95-fold (99% CI, 16-310) the expected one on the basis of age-sex-specific incidence rates in the general population of the same area. Altogether, about 25% of AIDS cases developed a cancer during HIV infection. In spite of the small size of the present study, the results confirm the role of population-based cancer registries in the assessment of the occurrence of malignancies in AIDS patients.