Data were obtained on patients' attribution of symptoms using two different post-marketing surveillance methods. In the patient-initiated method, outpatients were randomly assigned to have a printed notice conspicuously attached to the outside of their medication bags; the "outsert" requested the patients to monitor themselves during the next 2 weeks and to report via a toll-free telephone number any new or unusual symptoms. In the staff-initiated method, other patients filling the same target drug prescriptions did not receive any outsert but were identified at the pharmacy for a telephone interview 10 to 14 days later. The target drugs were chosen from two classes for which the major adverse drug reactions (ADRs) are well identified: oral antibiotics and tricyclic antidepressants. During the interview, for each reported symptom the patient was asked whether it might have been caused by the target drug. Results indicated that older patients, irrespective of the surveillance method or the drug class, appear to be capable of discriminating probable ADRs at least as well as or possibly better than younger patients.