Objective: To assess the implementation of a pharmacist intervention program within an emergency department, including the analysis of interventions by the pharmacist and identification of medication problems in the department. Methods: A pharmacist was employed part-time in the emergency department for 7 weeks starting in February 2009. Patients with a patient-related or drug-related risk factor or who were likely to be admitted to hospital were included from the areas of the emergency department's cubicles, observation unit, and/or critical case section. The variables recorded were demographic characteristics, clinical data of interest, medical history, medications currently used, reasons for seeking care, and drugs prescribed in the emergency department. Pharmacist interventions related to a medication problem were recorded and classified. Other information gathered included the sequence of therapeutic recommendations and adverse drug interactions as a reason for seeking emergency care. Results: Ninety patients (63.7% women) with a mean (SD) age of 79.3 (10.6) years were studied. One hundred thirty-one interventions were recorded for 62 (68.9%) patients. One hundred twenty of these (91.6%) involved a medication problem. Initiating a treatment was the most frequent decision (in 54 [45%] patients); in 28 cases (23.3%) a medication was withdrawn. Regarding types of medication problems detected, 53% were related to need for a drug, 15% to efficacy, and 31.7% to safety. The prescription of more than 5 drugs was a significant risk factor for requiring the pharmacist's intervention (P=.008). The presence of 2 or more risk factors was associated with a medication-related problem (P=.0032). The evaluation of drugs according to type of activity showed that most (48 [36.6%]) were acting on the cardiovascular system. Conclusions: Employing a pharmacist in the emergency department facilitates the detection and resolution of medication problems. The most frequently encountered problems were related to a patient's usual medication regimen (whether for a drug that was needed but not prescribed or a drug that was prescribed but not needed for the patient's current situation). The patients who benefited most from the pharmacist's intervention in terms of efficacy and safety were those currently taking 5 or more medications. [Emergencias 2010;22:85-90]