One of the most important aspect of any surveillance system is completeness. We conducted an evaluation of the completeness of the AIDS surveillance system (SSAIDS) of Lazio region. We estimated the underreporting of AIDS cases in SSAIDS of Lazio in the period 1996-1998 through a linkage with the regional Hospital Information System (HIS) for the same years. A logistic regression model was used to estimate the relative risk (Odds Ratios--OR) of being unreported by demographic and clinical characteristics. A total of 56 cases had not been reported in the period 1996-1998, corresponding to 3.6% of underreporting. The risk of not being reported was associated with undetermined value of CD4 at diagnosis of AIDS (OR 15.40), unknown modality of exposure (OR 3.15) and with AIDS defining illness at diagnosis (ADI) as mycobacteriosis (OR 11.65), encephalopaties (OR 4.47) and toxoplasmosis (OR 8.67). Differences were also observed by clinical centre of diagnosis. In conclusion, the degree of underreporting is lower in our region than in other countries and the SSAIDS has a high completeness in Lazio. In agreement with other studies, underreporting is more frequent for cases with undermined value of CD4 at diagnosis of AIDS and unknown modality of exposure. The results regarding ADI need further investigation.