Gastroesophageal reflux disease (GERD) is one of the most common esophageal diseases in developed countries. It is widely believed that GERD symptoms are caused by acid refluxate within the esophagus, so ambulatory 24 hour pH-monitoring became the gold standard in detecting gastroesophageal reflux. Traditional ambulatory pH monitoring is unable to detect a gastroesophageal reflux with pH >4. The introduction of multichannel intraluminal impedance and pH (MII-pH) brought new possibilities in detecting GERD. In this technique impedance identifies reflux episode whereas pH sensor further characterizes it as either acid (pH <4) or non-acid (pH > or =4). This is a great progress in diagnosing GERD but MII has also some imperfections related to pathological changes in the esophageal mucosa such as esophagitis or Barrett oesophagus, which are connecting with a very low baseline impedance values. Changes in the esophageal mucosa may also impair the esophageal motility and esophageal transit leading to some fluid retention in the esophagus. It should be stressed that very low impedance baseline creates a difficulty in interpreting the MII-pH study. In such a case it might be almost impossible to interpret the study as the interpreter does not see characteristic drop in impedance progressing either orally (reflux episode) or swallow but only almost flat impedance lines. Therefore, future studies are needed to further evaluate this problem.