Introduction and aim: Functional abdominal pain (FAP) is one of the major astrointestinal complaints in childhood. Studies have reported occult constipation (OC) as one of the leading causes of abdominal pain. Recent researches have proposed laxatives as potent therapeutic targets for abdominal pain in patients with OC. However, no study has compared effect of poly ethylene glycol (PEG) and lactulose on occult constipation. Materials and methods: 51 patients aged 4 to 18 years with abdominal pain who had OC (defined as fecal impaction in abdominal X ray) were studied. Demographic and clinical data including age, sex, body weight, height, abdominal pain duration, abdominal pain rate and fecal odor were registered. They were randomly assigned to receive PEG (1gr/kg) or Lactulose (1cc/kg) for at least two weeks. All patients were reevaluated by pain measurement scale after at least two week...