We report the first results of a comparison between the Italian Registry on AIDS (RAIDS) and 13 population-based cancer registries (about 8 million population in 1991) with respect to the notification of Kaposi's sarcoma and non-Hodgkin's lymphoma. Routine indicators of data quality and completeness have been found in both types of registry, consistent with the best international standards. A linkage process was carried out on about 339,000 cancer notifications and 3,134 AIDS notifications and was herein restricted to individuals under the age of 50. Out of 243 Kaposi's sarcomas at either type of registry, 90 (37%) were reported as such by both; 68% of individuals with Kaposi's sarcoma at cancer registries could be identified at the AIDS registry, including AIDS-defining illnesses other than Kaposi's sarcoma; 62% of individuals with Kaposi's sarcoma at RAIDS could be found at cancer registries. Of 2,104 non-Hodgkin's lymphomas at either type of registry, 55 were reported as such by both; 65% of individuals reported as having non-Hodgkin's lymphoma at the AIDS registry were found at cancer registries. Our present results indicate the scope for improving cancer assessment in individuals with HIV infection and AIDS and the potential of AIDS and cancer registries for such a purpose.