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Acid clearing, the interval while intraesophageal pH is 1 unit fall while pH < 4) detected only 35% of acid rereflux REs. When a variety of patients and study conditions was examined, simultaneous manometry–pH monitoring found more frequent acid rereflux in the following situations: (1) patients with vs those without esophagitis; (2) recumbent vs upright posture, and (3) postprandial vs preprandial. Of pathophysiologic importance, acid rereflux in the blind spot is the most common cause of prolonged daytime acid REs in GERD patients. Of clinical importance, the 24-hr pH parameter “% acid exposure” should be relied upon most in interpreting the 24-hr pH record, because those parameters that relate to RE frequency may be inaccurate due to acid rereflux REs that are not counted. Furthermore, identifying as many REs as possible gives a more reliable indication of the severity of antireflux barrier incompetence, as well as more REs to correlate with patients symptoms that should improve sensitivity of the symptom index. Ambulatory simultaneous pH monitoring and MII will allow these and other roles for acid rereflux to be assessed during the patients normal day.