Caffeine affects many aspects of body function including the gastrointestinal system. A single-blinded experimental study was performed to evaluate the effect of caffeine on lower esophageal sphincter (LES) and esophageal peristaltic contractions in healthy Thai adults. The volunteers were six men and six women aged 19-31 years. Subjects drank 100 mL of water. Five wet swallows were performed 30 min after the drink. The basal LES pressure was continuously measured using esophageal manometric technique. They then consumed another 100 mL of water containing caffeine at the dose of 3.5 mg/kg body weight. The swallows and basal LES pressure monitoring were repeated. The results showed no change in basal LES pressure after a water drink while caffeine consumption significantly lowered the pressure at 10, 15, 20 and 25 min. The mean amplitude of contractions and peristaltic velocity were decreased at the distal esophagus at 3 and 8 cm above LES. The mean duration of contraction was decreased at the distal part but increased at the more proximal esophagus. The heart rate, systolic and diastolic blood pressures were increased significantly at 10-20 min after caffeine ingestion. This study indicated that caffeine 3.5 mg/kg affected esophageal function, resulting in a decrease in basal LES pressure and distal esophageal contraction, which is known to promote the reflux of gastric contents up into the esophagus.