Associations between ambient sounds and accuracy of pharmacists' prescription-filling performance in a pharmacy was studied. Pharmacists were videotaped as they filled prescriptions each workday for 23 days. Each filled prescription was inspected by the investigator. Deviations from the physician's written order were considered errors. Videotape analysis was used to detect unpredictable, predictable, uncontrollable, and controllable sounds. A within-subjects case control study design was employed to determine whether the frequency of ambient sounds was significantly different when prescriptions with errors, compared with those without errors, were filled. Loudness, in terms of equivalent sound levels (Leq) for each half hour, was analyzed for a relationship to dispensing error rate. A mean dispensing error rate of 3.23% was found. Unpredictable sounds, controllable sounds, and noise had a significant effect on pharmacists which resulted in a decreased dispensing error rate. These results suggest that the quality of pharmacists' performance is not adversely affected by ambient sound. As equivalent sound levels increased, the error rate increased to a point, then decreased.