The extent of potential medication-related problems was examined using a thorough review of drug therapy for 53 elderly patients who averaged five chronic illnesses and who used a mean of 11 drugs. An average of 11 specific potential medication-related problems was identified for each patient. These problems fell into three broad categories: drug toxicity, physician prescribing, and patient medication behaviors. The strongest predictor of the total number of potential problems was the number of prescription medications. The type of drug therapy review used in this study can help health professionals identify and prevent the types of medication-related problems occurring in multiple medication users.