There is evidence that gastro-oesophageal reflux can cause apnoea in infancy and chronic respiratory symptoms in children of all ages. This study aimed to investigate its importance in infants with chronic respiratory symptoms; infants with other symptoms were also studied to clarify the relationship. Fifty infants with chronic respiratory symptoms, 32 with apparent life threatening events and 25 with vomiting were studied. Forty-seven, 29 and 23 respectively underwent a twenty-four hour pH study for assessment of gastro-oesophageal reflux and 44, 16 and 7 had their respiratory function measured. For all these and 23 healthy controls a family background and symptom questionnaire was completed. A comparison of the results of pH studies and respiratory function tests between the three groups was made and the possible influences of background characteristics were assessed. The relationship between the results of pH studies and respiratory function tests within the groups was also investigated. Abnormal gastro-oesophageal reflux was found in all three groups but particular patterns of reflux were not associated with specific clinical manifestations. Infants with a family history of atopy had less gastro-oesophageal reflux. Infants whose mothers smoked in pregnancy had more abnormal respiratory function tests. There was no correlation between the degree of abnormality of pH studies and respiratory function tests although individual infants' symptoms seemed to be temporally linked. The effect of anti-reflux treatment was assessed in infants with respiratory symptoms and vomiting. This did not significantly improve gastro-oesophageal reflux, thus precluding assessment of any concomitant improvement in respiratory function tests. Further studies were undertaken to determine the reproducibility of pH study results. Considerable differences in consecutive pH studies were demonstrated and were related to biological rather than technical variation. These differences make it more difficult to demonstrate a treatment effect and may obscure patterns of reflux associated with symptoms.