Although drug-related problems are a major source of morbidity, the literature provides little information regarding utilization of ambulatory care resources for drug-related problems. OBJECTIVE: The purpose of this study was to examine the nature and extent of the use of ambulatory care services due to adverse effects of medications in the United States. METHODS: This study analyzed patient records abstracted from the 1996 National Ambulatory Medical Care Survey and the National Hospital Ambulatory Medical Care Survey. Patient visits resulting in principal diagnoses of adverse effects of medications (ICD-9-CM E-code 930.00-947.9) were analyzed. The resource utilization and demographics associated with these visits were examined using descriptive statistics. RESULTS: During 1996, 0.31% of the visits to ambulatory settings were due to adverse effects of medications, representing an estimated 2.73 million visits per year or 1.03 visits per 100 persons. The majority (87.64%) were office-based visits. Emergency and outpatient department visits were 7.97% and 4.39%, respectively. Visit rates for drug-related problems were highest in whites, females, patients over 74 years old, and patients in the West. The therapeutic agents most often responsible for these visits were antibiotics (20.36%), cardiovascular drugs (10.66%), and hormones and synthetic substitutes (7.68%). The most frequently cited primary reasons for the visits were skin rash (9.89%), cough (8.56%), and adverse effect of medications (6.52%). Other than blood pressure and blood tests, few diagnostic or screening services were utilized for the patients. The medications most frequently prescribed to treat these problems were corticosteroids, antihistamines, and drugs for GI disorders. The majority (81.54%) of the drug-related visits included instructions for a return visit and 1.40% of the drug-related visits resulted in hospital admission. CONCLUSION: Utilization patterns due to drug-related problems can pose a significant burden on ambulatory care resources. Pharmaceutical care can play an important role in reducing these problems.