Traditionally, patients with suspected or known and possible recurrent Crohn's disease have been investigated by small bowel barium radiology, which incurs a relatively high radiation dose. Despite patient selection a significant number have a normal barium examination. A prospective study was performed to evaluate the use of transabdominal ultrasound as the initial investigation in these two groups. One hundred and twenty-seven patients were examined, including 31 with a previous history of Crohn's disease. In the known Crohn's disease group there were 14 true positive ultrasound examinations and eight true negative, with six false positive and three false negative examinations. In the 96 patients not previously known to have Crohn's disease, there were 18 true positive and 70 true negative examinations, with two false positive and six false negative examinations. The overall sensitivity for ultrasound was 78% with a specificity of 91%. A significant learning curve was apparent in the early stages of study; in the last 64 patients the sensitivity was increased to 87%. These data support the use of ultrasound as the initial investigation in patients with suspected Crohn's disease or recurrence, prior to consideration for small bowel barium radiology, to reduce the large number of unnecessary small bowel barium examinations currently being performed.