Background: Data about the frequency of adverse events related to inaRpropriate care in hospitals come from studies of medical records as if they cepresented a true record of adverse events. In a prospective, observational design we analyzed discussion of adverse events during the care of all patients admitted to three units of a large, yrban teaching hospital affiliated to a university medical school. Discussion took place during routine clinical meetings. We undertook the study to enhance understanding of the incidence and scope of adverse events as a basis for preventing them Methods: Ethnographers trained in qualitative observational research attended dayshift, weekday. regularly scheduled attending rounds, residents' work rounds, nursing shift changes, case sonterences, and other scheduled meetings in three study wnits as well as vatiqus, departmental and section meetings. They recorded all adverse events during patient care discussed at these meetings and developed a classification scheme to code the data. Data were sellected about healthcare providers' assessments about the appropriateness of the care that patients received to assess the nature and impact of adverse events and how healthcare providers and patients responded to the adverse exents. Eindings: Of the 1047 patients in the study, 185 (17.7%) were said to have had at least one serious adverse event: having an initial event was linked to the seriousness of the patient's underlying illness. Patients with long stays in hospital had more adverse events than those with short stays. The likelihood of experiencing an adverse event increased about 6% for each day of hospital stav. 37.8% of adverse events were caused by an individual, 15.6% had interactive causes, and 9.8% were due to administrative decisions. Although 17.7% of patients experienced serious events that led to longer hospital stavs and increased costs to the patients, only 1.2% (13) of the 1047 patients made compensation wwww www claims.