Objective: To determine whether a prescription refill history obtained by telephoning patients’ pharmacies identifies poor adherence with asthma medications more frequently than physician assessment. Methods: The study population consisted of 116 children with persistent asthma who were Medicaid recipients; patients who received medication samples were excluded. During a clinic visit pulmonologists interviewed patients, caretakers, or both and estimated adherence on a checklist. A nurse asked the caretakers where they obtained medications and telephoned 66 identified pharmacies for refill histories. The maximum possible adherence was calculated as the number of doses refilled/number of doses prescribed × 100 for a mean duration of 163 days (range, 63 to 365 days). The accuracy of the refill information was determined from Medicaid reimbursement records. Results: Information provided by pharmacies was 92% accurate. The mean (95% CI) of maximum potential adherence was 72% (65%,77%) for theophylline, 61% (55%,68%) for inhaled corticosteroids, and 38% (23%,53%) for cromolyn; only cromolyn and theophylline were significantly different. Physicians were able to identify 21 (49%) of 43 patients who refilled ≤50% of prescribed doses of long-term symptom controllers and only 3 (27%) of 11 patients who used albuterol excessively. Conclusions: Physicians often were unable to identify patients with very poor adherence. Checking prescription refills is an accurate and practical method of identifying such patients. (J Pediatr 2000;136:532-6)