Characteristics of unclaimed prescriptions at a university hospital's ambulatory care pharmacy were determined, and reasons why prescriptions went unclaimed were identified. Over a 15-week period, 522 unclaimed prescriptions for 344 patients were identified. Patient and prescription data for these unclaimed prescriptions were obtained from pharmacy records. Telephone interviews were conducted with 157 patients to determine (1) why they had not claimed their prescriptions and (2) whether their treatment had been interrupted as a result. Of the unclaimed prescriptions, 224 were for discharge medications. Only 38 (17.0%) of the discharge prescriptions were eventually dispensed without pharmacy intervention, compared with 147 (49.3%) of the other unclaimed prescriptions. Patients' reasons for not claiming their prescriptions suggested that they had complied with the physicians' intentions for 72.6% of their prescriptions. The two most frequently reported reasons were that the prescription had been obtained elsewhere and the physician had changed or discontinued the regimen. Unclaimed prescriptions, especially for discharged patients, usually resulted from a lack of communication among patients, pharmacists, and other health care providers.