The objective of this natural history study is to assess the specific gastrointestinal (GI) symptoms and pathophysiology of chronic abdominal pain of unknown origin at the molecular level. This research will test the hypothesis that chronic abdominal pain of unknown etiology has an inflammatory mechanism. Although increased body weight may be associated with increased inflammation, it is unclear that increased body weight is associated with chronic abdominal pain. Accordingly both overweight and normal weight subjects with and without chronic abdominal pain of unknown origin will be included in this study. The study design is a longitudinal natural history.