Forty eight patients with moderate to severe asthma were enrolled in a double blind crossover study designed to evaluate the effects of ranitidine treatment, 150 mg twice daily for four weeks, on gastro-oesophageal reflux, asthma control, and bronchial reactivity. All 48 had a history of reflux symptoms and 27 had in addition reflux associated respiratory symptoms. Thirty two patients had objective evidence of acid reflux on 24 hour pH monitoring (pH of less than 4 for more than 1% of the 24 hours) and 27 patients had a positive result in the acid perfusion test. Reflux symptoms were significantly improved after ranitidine treatment. Ranitidine treatment was associated with modest improvements in nocturnal asthma and daily use of inhaled bronchodilator drugs but there was no significant change in bronchial reactivity, lung function, peak flow, or the number of eosinophils in the blood. Comparisons between the effect of ranitidine treatment on asthma control were performed between patients with and without a history of reflux associated respiratory symptoms, with and without a positive result in the acid perfusion test, and with and without objective evidence of gastro-oesophageal reflux. A history of reflux associated respiratory symptoms was the only factor that predicted an improvement in asthma control after ranitidine treatment. These results indicate that antireflux treatment will produce only small improvements in asthma control in asthmatic patients with a history of gastro-oesophageal reflux.