Intraesophageal pH recordings from 40 patients with abnormal studies who underwent fundoplication during a 1-year period were evaluated. Scores were recalculated for the first 12, 16, and 20 hours, respectively. Five of the recordings were for less than 24 hours, but were grossly abnormal and mathematically could not have normalized at 24 hours. The percentage of studies which were normal at 12, 16, and 20 hours were 20%, 15%, and 0%, respectively. One fifth of the patients would have been denied a fundoplication based on 12-hour results. In addition, 75 consecutive pH recordings were evaluated prospectively. Total scores were determined for the first 12, 16, and 20 hours with accuracies of 85%, 93%, and 92%, respectively. The sensitivity of 12-hour monitoring was only 78%. All abbreviated studies had a low rate of false positives; however, any false positives may be significant when used to determine the need for an operation. Abbreviated studies were associated with high false-negative rates (up to 52% for the 12-hour studies) for the calculated mean duration of sleep reflux, a parameter that has been found to be useful in identifying patients with respiratory symptoms secondary to gastroesophageal reflux. The use of abbreviated pH monitoring is discouraged.