GERD (gastro esophageal reflux disease) is defined as a condition that develops when the reflux of stomach contents causes troublesome symptoms and/or complications. Endoscopic-positive GERD can be easily diagnosed with endoscopy, while endoscopic-negative GERD cannot be. PPI test, which reveals the disorders by judging symptom-relief after PPI administration, is an effective tool for diagnosis of NERD, and extraesophageal GERD such as LPRD and bronchial asthma. Diagnostic power of PPI test is limited owing to the low PPI's cure rate against NERD, about 40%. PPI test-negative NERD is considered as non-acid associated NERD. Most of the NERD patients have the symptoms of functional dyspepsia(FD) for which the most effective medication is PPI administration, leading to the notion that subgroup of GERD and FD is considered as an acid associated disorder. This diagnostic entity is practical in a sense that anti-acid treatment is very effective for this disease. Besides, PPI test is a very useful tool to differentiate acid associated disorder from GERD and/or FD.