Objectives: To determine the frequency of potentially inappropriate medication selection for older persons presenting to the ED, the most common problematic drugs, risk factors for suboptimal medication selection, and whether use of these medications is associated with worse outcomes. Methods: The authors performed a prospective cohort study of 898 patients 65 years or older who presented to an urban academic ED in 1995 and 1996. Seventy-nine percent of the patients were African-American and 43% did not graduate from high school. Potentially inappropriate medications and adverse drug-disease interactions were identified using the 1997 Beers explicit criteria for elders. During the three months after the initial visit, revisits to the ED or hospital, death, and changes in health-related quality of life were analyzed as measured by validated questions adapted from the Medical Outcomes Study. Results: Upon presentation, 10.6% of the patients were taking a potentially inappropriate medication, 3.6% were given one in the ED, and 5.6% were prescribed one upon discharge from the ED. The most frequently prescribed potentially inappropriate medications in the ED were diphenhydramine, indomethacin, meperidine, and cyclobenzaprine. Emergency physicians added potentially inappropriate medications most often to patients with discharge diagnoses of musculoskeletal disorder, back pain, gout, and allergy or urticaria. Potentially adverse drug-disease interactions were relatively uncommon at presentation (5.2%), in the ED (0.6%), and on discharge from the ED (1.2%). Potentially inappropriate medications and adverse drug-disease interactions prescribed in the ED were not associated with higher rates of revisit to the ED, hospitalization, or death, but were correlated with worse physical function and pain. However, confidence intervals were wide for analyses of revisits and death. Conclusions: Suboptimal medication selection was fairly common and was associated with worse patient-reported health-related quality of life.