Among previous cases of mediastinal pseudocyst requiring surgical decompression, all but one had been found at surgery to occupy a position both in the mediastinum and in the upper abdomen. In the present case, although preoperative ultrasound and CT scans suggested that the pseudocyst was straddling the diaphragm, an abdominal portion could not be found at surgery, and the pseudocyst was drained successfully through the diaphragm by a Roux-en-Y loop of jejunum. Because ultrasound and CT scan may not be able to determine the precise relationship of a mediastinal pseudocyst to the diaphragm and the availability of the lower portion of the pseudocyst for surgical decompression, an endoscopic retrograde cholangiopancreatography is strongly recommended as part of the preoperative evaluation.