BACKGROUND Transient lower esophageal sphincter relaxations (TLESRs) are the major mechanism permitting gastroesophageal reflux (GER). Little information is available on how anti-reflux surgery affects reflux mechanisms, especially TLESRs. We evaluated the effects of partial fundoplication (Belsey Mark IV) on reflux mechanisms. METHODS Sixteen patients were prospectively studied before and after Belsey Mark-IV operation by endoscopy, 24-h esophageal pH-metry, and simultaneous recording of pH and lower esophageal sphincter (LES) characteristics by sleeve manometry. RESULTS The operation was successful in 14 of 16 patients (87%). Fasting and postprandial reflux decreased significantly (P < 0.01) after the operation. Partial fundoplication significantly (P < 0.05) decreased the number of TLESRs per hour in the fasting and postprandial period from 3.2+/-0.4 and 5.6+/-0.5 to 1.7+/-0.3 and 2.8+/-0.4, respectively. The percentage of TLESRs associated with reflux also decreased significantly (P < 0.05). Basal LES pressure increased from 14.7+/-2.1 mmHg to 17.9+/-2.6 mmHg (not significant). CONCLUSIONS Partial fundoplication controls GER through a reduction in the number of TLESRs and by decreasing the number of relaxations associated with reflux.