Generally accepted manometric criteria for the diagnosis of achalasia are absent peristalsis and incomplete lower esophageal sphincter (LES) relaxation. However, in some patients with otherwise typical features of achalasia, esophageal manometry shows complete LES relaxation during swallowing. To establish whether such apparently complete LES relaxations are functionally adequate, we quantified changes in resistance to flow at the esophagogastric junction (EGJ) during wet swallowing. We studied seven achalasia patients with manometrically complete (>80%) LES relaxation, eight achalasia patients with incomplete (<40%) LES relaxation, and eight healthy volunteers. Complete LES relaxation on standard manometry (open-tip catheters) was confirmed in five of the seven achalasia patients by a Dentsleeve. Changes in EGJ resistance to flow were quantified using a pneumatic resistometer. Manometrically, the relaxation time span was significantly longer in patients with complete LES relaxation than in those with incomplete relaxation (7.3 ± 0.5 vs. 4.4 ± 0.7 s; P < 0.05). The fall in EGJ resistance from basal values during swallowing was markedly reduced in both achalasia groups (21 ± 8% in those with manometrically complete relaxation and 4 ± 2% in those with incomplete relaxation) by comparison with healthy individuals, in whom resistance fell by 90 ± 3% (P < 0.05 vs. both achalasia groups). The duration of EGJ resistance drop was also much shorter in achalasia with (0.7 ± 0.2 s) and without (0.2 ± 0.1 s) complete LES relaxation compared with healthy control values (6.6 ± 1.2 s). Our results reveal that the apparently complete LES relaxation observed manometrically in some patients with achalasia is functionally inadequate since it is not associated with the normal profound fall in EGJ resistance to flow.