Gastro-oesophageal reflux occurs twice as much during transient lower oesophageal sphincter relaxations (TLOSRs) in patients with gastro-oesophageal reflux disease (GORD) compared to healthy volunteers (HVs). Our aim was to assess whether the localisation of the postprandial acid pocket and its interaction with a hiatal hernia (HH) play a role in the occurrence of acidic reflux during TLOSRs. Ten HVs and 22 patients with GORD (12 with HH<3 cm (s-HH), 10 with HH > or =3 cm (l-HH)) were studied. The squamocolumnar junction and diaphragmatic impression were marked with a radioactively labelled clip. To visualise the acid pocket, (99m)Tc-pertechnetate was injected intravenously and images were acquired up to 2 h postprandial. Concurrently, combined manometry/impedance and four-channel pH-metry were performed, with pH pull-through at multiple time-points. The rate of TLOSRs and the per cent associated with reflux was comparable between all groups. However, acidic reflux was significantly increased in patients, especially in patients with l-HH. Acid pocket length was significantly enlarged in patients. Moreover, immediately before a TLOSR, the acid pocket was more frequently located within the hiatus or above the diaphragm in patients with GORD (s-HH, 54%; l-HH, 77%) compared to HVs (22% of TLOSRs). Acidic reflux was significantly increased when the acid pocket was located above the diaphragm in all groups compared to a sub-diaphragmatic localisation. The position of the acid pocket is largely determined by the presence of a HH. Entrapment of the pocket above the diaphragm, especially in patients with l-HH, is a major risk factor underlying the increased occurrence of acidic reflux during a TLOSR in patients with GORD.