Forty-one patients were followed up for a median 14 months after Nissen fundoplication. Patients were questioned regarding their symptoms and 83% considered the surgery a complete success. Oesophageal manometry was performed before and after surgery. Median pre-operative lower oesophageal sphincter pressure (LOSP) was 5.5 mmHg; following operation median LOSP was 14 mmHg. The ability of the lower oesophageal sphincter (LOS) to relax with swallow was impaired by fundoplication; the median residual lower oesophageal sphincter pressure (RLOSP) was 0 mmHg prior to surgery and 3.5 mmHg following operation. Fundoplication appears to superimpose a zone of constant pressure on the lower oesophageal sphincter and this may be a further factor responsible for the efficacy of surgery.