The aim of this study was to assess the data quality of postoperative infections in a hospital discharge registry in the Emilia-Romagna region of Italy. Data from a prospective regional study of postoperative infections in 6158 patients from 31 of the 36 public hospitals of the region were compared with data from the regional hospital discharge registry, using different classes of ICD-9-CM codes. The sensitivity of the hospital discharge database for postoperative surgical infections was 10% when ICD-9-CM codes directly indicative of postoperative infectious complications were used. When non-specific codes of postoperative complications, not necessarily of infectious origin, were added, the sensitivity reached 21%. At present, the hospital discharge registry is not suited for surveillance of hospital-acquired infection.