Acid clearing, the interval while intraesophageal pH is < 4 after a traditional acid reflux event (RE), is a potential "blind spot" during pH monitoring, when reflux of acidified gastric contents may occur undetected by the pH probe. This is termed "acid rereflux." Acid rereflux comprised 61% (169/262) of acid REs in recumbent postprandial patients with severe GERD in two reports using simultaneous pH monitoring and manometry as well as multichannel intraluminal impedance (MII) in one, and scintigraphy in the other. Acid rereflux events often recurred with short intervals between them. The pH probe alone was insufficient to detect most acid rereflux REs, since expanding pH criteria for an acid RE (> 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.