Gastroesophageal reflux (GER) has been reported in a high percentage of children with asthma. To investigate the relationship between GER and nocturnal asthma (NA), 25 pediatric patients with histories of NA were studied prospectively with three to 13 weeks of in-patient observation to document NA and quantitative day-time and night-time wheezing frequency. Two distinct groups of patients were identified: 17 patients with predominant day-time asthma (DA) and eight with prominent NA. All were studied with overnight esophageal pH recordings to detect GER during sleep while also monitoring clinical status. A reflux score (RS) was calculated to quantitate the severity of GER. Sixteen of 25 patients (64%) had abnormal GER, including all eight patients with NA. A significant positive correlation was found between the RS and the percentage of nights with wheezing, while age, serum theophylline level and pulmonary function at the time of study did not correlate with RS. The NA group also had significantly higher RS than the DA group. Three of the eight NA patients had the onset of clinical wheezing during an episode of GER. We conclude that there is a significant association between NA and GER independent of age, lung function and theophylline levels and that GER may induce NA in a subpopulation of patients with reactive airways.