Background Adverse drug events (ADEs) are one of the most frequent causes of patient harm resulting from medical interventions, especially among inpatients. This study aimed to evaluate the incidence of ADEs and characterise them in terms of degree of harm, medication implicated and patient symptoms, at a Brazilian university hospital. Methods This is a retrospective study of chart review. The method, developed by the Institute for Healthcare Improvement, uses triggers to identify possible ADEs. The study population comprised adult inpatients at least 15 years old. Obstetric patients and those hospitalised for less than 48 hours were excluded. Time spent in the intensive care unit was not considered for the purposes of this study. Patients were selected on the basis of simple random sampling of records of patients discharged from January to July 2008. The records selected were reviewed by a multidisciplinary team. The indicators of ADE incidence were patients with ADEs and ADE rate per 100 patients. Patients with and without ADE were compared in the bivariate analysis. To identify the drugs classes most often associated with events, the number of prescriptions of each class of drug was related to the number of events assigned to it. Results The 240 inpatients studied were of mean age 50.8 (SD = 20.0) years, and mostly male (63.8%). A total of 44 ADEs were identified in 35 patient records, with 14.6% of patients presenting ADE and a rate of 18.3% ADEs per 100 patients. The most frequent were skin rash and nausea and vomiting, but severe ADEs were also identified. In the bivariate analysis long hospital stay and use of 10 or more drugs were associated with the occurrence of ADEs (p-value < 0.01). The drug classes associated with the highest number of events were antiinfective. Conclusion About 1/6 of the hospitalized patients in a teaching hospital showed adverse events what is, by itself, cause for concern. Increased number of prescribed drugs and greater period of hospitalization appear to favour the occurrence of these events. In the future studies with higher number of patients may offer evidences of the association. [ABSTRACT FROM AUTHOR]