Objective: To evaluate completeness of reporting of cases of AIDS to the Communicable Disease Surveillance Centre (CDSC) between 1982-1991. Setting-Southside of the Bloomsbury and Islington District Health Authority. Design-Reconciliation exercise with CDSC of cases reported with those known to have received treatment in the district from 1982 to March 1990. Case note review of unreported cases and follow-up at March 1991. Main outcome measures-Delayed and non-reporting of cases. Results: Cumulatively 13% (461351) of patients whose initial AIDS illness was diagnosed in the District remained unreported by March 1991. Non-reporting increased from 9%!. (2123) of cases diagnosed prior to 1985 to 28% (26192) of cases diagnosed between 1989 and 1990. After September 1987 the proportion ofpatients with a diagnosis of Pneumocystis carinii pneumonia or Kaposi's sarcoma was significantly higher in the reported group than in the non-reported group: 78% (124/158) v 51% (24149) p < 0.001. Nine of 19 (47%) cases of AIDS transferring their care into the district had not been reported by their previous District Health Authority. Conclusions-Within the district nonreporting of cases of AIDS has risen over time as the numbers of patients treated has increased. The physician must be aware of the full AIDS case definition for surveillance purposes and the implications ofnon-reporting for the allocation of special "ring-fenced" resources for AIDS care. Adequate investment in information and reporting systems would seem essential.