Objective. —To assess the completeness of acquired immunodeficiency syndrome (AIDS) case reporting in New York City (NYC), and to determine whether the completeness of reporting differs in various populations. Design. —Retrospective record review of hospital laboratory logs, death certificates, hospital discharge records, and patient registries at private physicians' offices and hospital outpatient clinics. Setting. —Public and private hospitals, and private physicians' offices in NYC. Patients. —Adults and adolescents with AIDS or with illnesses suggestive of AIDS were identified using both population-based and nonrandom sampling techniques. These persons were matched with the NYC AIDS case registry, and the medical records of nonmatching persons were reviewed to determine whether they met the 1987 Centers for Disease Control and Prevention surveillance case definition for AIDS. Main Outcome Measures. —The completeness of reporting was calculated for the five individual projects and for the aggregate database by gender, race/ethnicity, risk, borough of residence, age, first AIDS diagnosis, and year of diagnosis. Results. —Of 7015 persons with AIDS identified in the five projects, 5912 (84%) had been previously reported (range, 77% to 89%). The completeness of reporting ranged from 81% to 87% in all major gender, race/ethnicity, risk, borough of residence, and age subgroups. In a multivariate analysis, the odds of being unreported were significantly higher among outpatients in hospital clinics, out-of-state residents, persons with diagnoses other than Pneumocystis carinii pneumonia, and persons recently diagnosed with AIDS. Conclusions. —This study indicates that the NYC AIDS surveillance system functioned effectively during the first decade of the AIDS epidemic. Completeness-of-reporting studies are an integral part of AIDS surveillance, providing data that are critical for assessing the validity of the AIDS surveillance database. (JAMA. 1993;269:2995-3001)