Ambulatory 24-h pH monitoring in the distal oesophagus was performed in seven patients with erosive or ulcerative reflux oesophagitis to compare the effects of omeprazole and ranitidine in the management of gastro-oesophageal reflux disease. In a double-blind, crossover study patients were treated with either 60 mg o.m. omeprazole or 150 mg b.d. ranitidine. The pH measurements were performed before treatment and on the fourteenth day of treatment with either regimen. The total acid exposure time (percentage total time pH less than 4) was abnormal in six out of seven patients before treatment. During treatment with omeprazole the acid exposure time of five patients was normal in comparison with only two patients during ranitidine therapy. However, even with a rather high dose of omeprazole, pathological gastro-oesophageal reflux may still occur.