Decreased swallow frequency and low-amplitude or nonconducted primary peristaltic contractions are reported to prolong acid clearing in gastroesophageal reflux disease (GERD) patients. The aim of this study is to investigate which of these, or other factors, have a dominant role in long-duration pH reflux events (pHRE). Simultaneous manometry and pH monitoring was performed for 40 min before and after (beginning 40 min postprandial) a test meal. We arbitrarily chose 180 sec to divide pHREs into long or short pHREs. Twenty GERD patients with and without esophagitis were studied. Esophagitis patients had threefold more long pHREs than patients without esophagitis. In most (56%) long pHREs, additional reflux events during acid clearing was the only finding. Only 11% of long pHREs had either a decreased swallow rate (3%) or decreased peristaltic contraction amplitude (8%), as the only finding contributing to poor acid clearing. However, 18% of long pHREs had one of these peristaltic dysfunctions in combination with additional reflux events prolonging acid clearing. Only 15% of long pHREs had no apparent reason for poor acid clearing. In interpreting 24-hr pH monitoring, one should not assume prolonged acid clearing is due to peristaltic dysfunction; instead, it is often due to additional reflux events.