Summary: To evaluate the completeness and accuracy in the reporting of AIDS-associated Kaposi's sarcoma (KS) and non-Hodgkin's lymphomas (NHL) in Italy, a linkage study of the notifications to the Italian AIDS Registry (RAIDS—the national compulsory AIDS surveillance system) and the clinical and pathological records of the Italian Cooperative Group on AIDS-related tumors (GICAT—a nationwide voluntary reporting system for HIV-infected individuals who develop cancer) was carried out. A total of 288 cases of KS and 258 cases of NHL fulfilling the AIDS definition criteria, histologically diagnosed by the GICAT centers between January 1987 and March 1992, were matched with the 16,860 AIDS cases reported to the RAIDS up to March 1993. The linkage procedure, based on name, gender, and date of birth, identified 276 cases of KS (96%) and 241 cases of NHL (93%) present in both files (“linked” cases). The diagnosis of KS did not appear among the clinical manifestations of AIDS in the RAIDS file in 67 out of the 276 linked KS (24%), either with coincident dates of KS diagnosis and of case notification (19 cases) or when the KS diagnosis followed by ≤2 months the case notification to RAIDS (48 cases). Of the 241 linked NHL, 84 (33%) had no such neoplastic complications of AIDS listed in the RAIDS file, 23 with coincident dates of NHL diagnosis and of case notification and 61 with the NHL diagnosis made ≤2 months after the notification. A noteworthy discrepancy in the classification of the three histologic subtypes of NHL emerged between the GICAT and the RAIDS. The degree of underreporting of AIDS-associated cancers that emerged from the present study suggests that augmentation with other sources of oncological information is important to better estimate the burden of AIDS-related tumors and to study the interaction between HIV infection and cancer.