A manometric method to measure the effect of contraction of the crural sling of the diaphragm on intraoesophageal pressure is described. The manometric crural pressure was measured in 57 patients with gastro-oesophageal reflux disease, documented by 24-h ambulatory pH monitoring, and compared between patients with and without hiatal hernia. Repeat measurements were made in 33 patients who underwent antireflux surgery that incorporated a crural repair. Mean crural pressure in 41 patients with hiatal hernia was 5.0 mmHg, compared with 15.0 mmHg in 16 without hiatal hernia (P < 0.01). In the 33 patients undergoing antireflux surgery, mean crural pressure rose from 7.1 mmHg before operation to 11.6 mmHg afterwards (P < 0.02) overall, and from 4.7 to 10.1 mmHg (P < 0.01) in the 26 patients with hiatal hernia. These results confirm a measurable contribution of the diaphragmatic crural sling to resting pressure at the high-pressure zone. The crural pressure is deficient in patients with hiatal hernia compared with that in those with reflux but no hernia. The mean crural pressure is increased in patients by antireflux surgery, particularly in those with hiatal hernia. The results provide objective support for the role of the crural diaphragm in the antireflux mechanism and the rationale of performing crural repair during antireflux surgery, particularly in the presence of hiatal hernia.