UNLABELLED: Noncardiac chest pain (NCCP) presents as a frequent diagnostic challenge, with patients tending to use a disproportionate level of health care resources. Gastroesophageal reflux disease (GERD) is the most frequent cause of NCCP. GOALS: To test the efficacy of a potent acid-suppressing agent as a diagnostic test in the evaluation of NCCP and to compare it with three commonly used tests. STUDY: Eighteen men and 24 women, aged 22 to 77 years, who presented with recurrent chest pain complaints of a noncardiac etiology, as determined by rest/stress perfusion imaging with technetium Tc99m sestamibi (MIBI), were enrolled in a prospective, double-blinded, placebo-controlled, crossover trial using high-dose omeprazole. Thirty-seven patients completed both arms of the trial. Findings were compared with those of endoscopy, manometry, and ambulatory 24-hour two-channel esophageal pH monitoring. All patients underwent initial diagnostic upper endoscopy, esophageal manometry, and 24-hour pH monitoring. Patients were then randomly assigned to either placebo or omeprazole (40 mg/d orally twice daily) for 14 days, washed out for 21 days, and then crossed over. Patient's symptoms were determined using a Visual Analogue Scale to measure the severity of chest pain before and after each period. RESULTS: Seventy-one percent of patients in the omeprazole arm reported improved chest pain, whereas only 18% in the placebo arm did. Abnormal results on manometry (20%), 24-hour pH monitoring (42%), or endoscopy with visual evidence of esophagitis (26%) were found less frequently. Combination of the three tests did not significantly increase their usefulness. In NCCP patients with GERD, as defined by positive results on a 24-hour pH test or presence of esophagitis on endoscopy, omeprazole treatment led to a response in 95% of patients, whereas 90% of GERD-positive patients treated with placebo did not respond. Of NCCP patients determined to be GERD negative, 39% responded to omeprazole. CONCLUSIONS: Omeprazole as a first diagnostic tool in the evaluation of MIBI-negative NCCP is sensitive and specific for determining the cause of NCCP. Endoscopy, manometry, and 24-hour pH monitoring were not only less sensitive in diagnosing NCCP, but they were significantly more expensive.