It is almost 50 years ago, when L.M. Bernstein and L.A. Baker wrote a paper on esophageal pain created by intra-luminal acid infusion. Their main purpose was to distinguish the chest pain due to esophagitis from cardiac pain. As former experiments had failed to show the direct roll of acid on chest pain, esophageal pain observed in esophagitis patients had been thought to be the result of esophageal wall extension. Bernstein showed that acid itself could cause chest pains in not only endoscopically positive esophagitis patients but also in pseudoesophagitis patients as Bernstein called. The simple and effective method he developed during his trial was called as Bernstein test with respect. Nowadays, more than half of the patients who suffer GERD symptoms are diagnosed as non-erosive reflux disease (NERD). Effective management of NERD is one of the most anticipated fields in the clinical upper GI treatment, and for that purpose, selecting the group of patients who are sensitive to acid is especially important. In this paper, I will describe about this Bernstein's "old but up-to-date" original paper in detail and consider its present-day interpretation. It has become obvious recently that acid is not the only cause of the esophageal pain of NERD patients. But, the importance of acid in NERD is still not small. Both as the theoretical basis of acid induced esophageal pain and as clinical method of measuring acid sensitivity in esophagus, Bernstein test should be recalled frequently and improved further.