A new patient-initiated, pharmacy-based postmarketing surveillance system is described. At the time a new prescription for a targeted drug was filled, 2705 outpatients (experimentals) randomly assigned to the new system had a printed notice attached to their medication bags: the information requested them to report any "new or unusual symptoms" during the next 2 weeks by a toll-free telephone number to a trained nonprofessional who conducted a standardized adverse drug reaction (ADR) interview. To help validate the new system, another sample of 1109 patients (controls) did not receive a request for self-monitoring but were interviewed by telephone 2 weeks later. Target drugs were chosen from two classes for which side effect profiles are well identified: oral antibiotics and tricyclic antidepressants. Results show that within both drug classes, all patient-initiated reports closely matched those obtained from controls; the experimental and control groups also reported predictably high relative frequencies for the most commonly expected ADRs. Additional analyses suggest that a patient-initiated monitoring system could prove to be a promising complement to existing physician-based surveillance systems.