We studied the sonographic findings in seven patients in whom afferent loop obstruction was first detected by sonography. All seven subsequently were proved at surgery to have afferent loop syndrome. The causes of the obstruction included internal hernia (n = 3), cancer recurrence (n = 2), marginal ulcer (n = 1), and development of cancer at the anastomosis site (n = 1). In all cases, the dilated afferent loop was seen on sonography as a tubular structure in the upper abdomen crossing transversely over the midline. The distal end of the afferent loop could be traced toward the anastomosis. The probable cause of the syndrome was predicted on the basis of sonography in two of three patients with cancer at the anastomosis. Our experience suggests that afferent loop syndrome can be diagnosed sonographically on the basis of the detection, location, and shape of the dilated afferent loop.