PURPOSE: To review the radiologic findings of retroanastomotic hernia and to derive useful US and CT criteria to assist in the diagnosis of the condition in patients who have undergone gastrojejunostomy. MATERIALS AND METHODS: During a recent eight-year period, we encountered 11 consecutive cases of retroanastomotic hernia. Of the patients involved, nine underwent ultrasound (US), eight underwent computed tomography (CT), and in three, small bowel follow-through imaging was performed. The US and CT scans were reviewed to determine abnormal findings; surgical proof was available in all cases. RESULTS: The efferent loop was herniated through the defect created behind the anastomosis in eight cases, both the efferent and afferent loop in two cases, and the afferent loop in one case. Retroanastomotic hernia was prospectively diagnosed in ten of these eleven cases. Among the eight cases of efferent loop herniation, US and CT signs of retroanastomotic hernia included whirling of mesenteric vessels, jejunal loops and mesentery in the periumbilical abdomen (8/8); mural thickening of herniated bowel loops (6/8); dilatation of herniated bowel loops (4/8); (at US) decreased peristalsis of herniated bowel loops (3/7); and (at CT) decreased contrast enhancement of herniated bowel loops (1/5). In one case, US and CT signs of retroanastomotic hernia of the afferent loop included its dilatation and whirling of a short length behind the anastomosis. In two cases, US and CT signs of retroanastomotic hernia of both the afferent and efferent loop included findings of both afferent and efferent loop herniation. CONCLUSION: Retroanastomotic hernia is an important and underdiagnosed condition, and the US and CT findings we have described may permit its accurate diagnosis.