A review of barium meal studies in 32 cases of congenital intrinsic duodenal obstruction (CDO) from the Adelaide Children's Hospital was performed to assess and measure pre and post operative duodenal dilatation. Comparisons were made with a group of 153 normal barium studies from the same Hospital with a view to quantifying normal and abnormal proximal duodenal calibre. Duodenal size was expressed as a ratio D1/L1, (diameter of 1st part of duodenum/height of first lumbar vertebra). In this study a normal range of duodenal diameter was found to be 0.90 to 1.62 (+/- 2 SD from mean), with a mean of 1.26. The CDO patients recorded a range of postoperative ratios on first follow-up barium studies of between 1.2 and 4.5 with a mean of 2.28. Where serial studies had been performed, duodenal diameter was found to show some decrease in size postoperatively. In patients who underwent surgery for membrane obstruction there was a trend towards normalisation of duodenal diameter over time, compared with the atresia group. However, this difference did not reach statistical significance.