OBJECTIVE Laparoscopic adrenalectomy is generally performed with carbon dioxide insufflation of the cavity and requires multiple trocars. This study reports the outcomes of retroperitoneoscopic adrenalectomy (RA) for adrenal tumors via a single port using a large cylinder without carbon dioxide insufflation. METHODS Fifty-four patients with adrenal tumors were treated using RA via a single large port. The average tumor size was 2.6 cm. For surgery, patients were placed in the lateral decubitus position with slight flexion, and a 4.5-cm skin incision was performed below the 12th rib in the midaxillary line. The retroperitoneal space was dissected using index fingers and a balloon dilator. A rectoscope tube with a 4-cm diameter was inserted, and the adrenal glands were removed endoscopically via the single large port without carbon dioxide insufflation. RESULTS This procedure was completed in 53 patients (98.1%). The average duration of surgery was 203 minutes, and the mean estimated blood loss was 252 mL. Four patients (7.4%) required blood transfusion. Postoperative major complications, including fulminant hepatitis and pulmonary thrombosis, were observed in two patients (3.7%), and the patient with hepatic disease died on the 14th postoperative day. The mortality rate after surgery thus was 1.9%. However, no local tumor recurrence or hormonal relapse has occurred at a median follow-up of 34 months. CONCLUSIONS This procedure appears to be effective and relatively minimally invasive. However, it is limited by the narrow working space and restriction of the manipulation of instruments.