Background Adverse drug events are considered determinants of patient safety and quality of care. Objective To assess the characteristics of adverse drug events in patients admitted to an intensive care unit and determine the impact of severity of illness and nursing workload on the prevalence of the events. Methods A cross-sectional survey based on retrospective analysis of a high-quality patient data management system for a university-based intensive care unit was used. The prevalence of adverse drug events was measured by using a validated global trigger tool adapted for the critical care environment. Severity was determined by using a validated algorithm. Disease severity and nursing workload were assessed by using validated scoring systems. An investigator blinded to the study and a panel of experts assessed putative serious adverse drug events for each drug taken. Characteristics of patients with and without adverse drug events were compared by using univariate and stepwise multivariate logistic regression. Results During 175 of 1009 intensive care unit days screened, 230 adverse drug events occurred in 79 patients. The most common events were hypoglycemia, prolonged activated partial thromboplastin time, and hypokalemia. Of the adverse events, 96% were classified as causing temporary harm and 4% as causing complications. Both mean severity of disease and nursing workload were significantly higher on days when 1 or more adverse drug events occurred. Conclusion Adverse drug events were common in intensive care unit patients and were associated with illness severity and nursing workload.