One of the most important aspects of any surveillance system is degree of completeness. We conducted a multiple source capture-recapture study using the 1994 Massachusetts Uniform Hospital Discharge Data Set (UHDDS) and Medicaid claims data to evaluate the completeness of the state's AIDS registry. We used encrypted social security numbers as the primary link to ensure confidentiality. For cases that did not link in the first round owing to missing social security numbers, we linked using gender and date of birth. Staff reviewed unmatched records from the Uniform Hospital Discharge Data Set and Medicaid datasets to determine subjects' AIDS case status. Using the Uniform Hospital Discharge Data Set, the AIDS registry was 92.6% complete (95% confidence interval (CI) = 91.6-93.5). The Medicaid claims dataset suggested the AIDS registry to be 94.5% complete (95% confidence interval = 93.7-95.3). The completeness of reporting to the state AIDS registry continues to be high, but there are differences by gender and mode of transmission of the virus. The continued assessment of completeness of reporting is important to ensure quality of the surveillance database over time.