Patients must understand their physicians' recommendations if they are to follow medical advice. This study assesses the degree to which patients and physicians share basic information about medications. Patients who regularly attended the medical clinics of a large, urban teaching hospital were asked to recall the identity, purpose, and dose schedule of medications that were prescribed for them. Patients' responses were compared to the medications actually prescribed by their physicians. Patients identified 90% of medications prescribed during the visits, knew the purpose 83%, and the correct dose schedule for 80%. Only 58% of patients knew the dosage schedule of all their medications correctly. Patients' knowledge of prescribed drugs was inversely related to the number of their medical problems and the number of medications prescribed. Lack of effective communication between physicians and patients about medications may be an important reason why patients do not follow medical advice.