The detection of adverse drug reactions (ADRs) by a traditional passive reporting system and by a method involving patient and provider interviews was studied. The study sample consisted of randomly selected outpatients seen by their primary care provider during scheduled appointments in January and February 2001 at a Veterans Affairs medical center. After ambulatory care clinic sessions, patients and providers were asked (by telephone and in person, respectively) to identify potential ADRs. Also obtained were demographic data, information about drug regimens, and the severity and management of each ADR. A standardized ADR-assessment tool was used to determine the severity of each reported reaction and its causal relationship with the medication. A total of 198 patients were included. Of these, 51 (26%) had one or more ADRs. The patient and provider interviews identified a total of 83 ADRs, compared with 1 ADR identified by the passive reporting system. When providers were made aware of the ADRs they had not identified, changes were made to the patient's medication regimen in 34% of cases. The risk of an ADR was not associated with age, number of medications, or provider type. Direct patient and provider interviews yielded a significantly higher rate of ADR detection in an ambulatory care setting than did a passive ADR-reporting system.