INTRODUCTION: Intestinal perforation is one of the most serious complications of colonoscopy. Its diagnosis and treatment may be delayed by the presence of unexpected symptoms. We present a case of pneumoretroperitoneum, pneumomediastinum, and subcutaneous emphysema that occurred due to retroperitoneal penetration by EMR at the ascending colon with the initial symptom as right cervical pain alone. CASE PRESENTATION: The patient is a 51-year-old man. Colonoscopy revealed LST of about 15 mm in diameter at the lower ascending colon (Figure. 1). Then, EMR was performed. After EMR, the patient complained of mild right cervical pain. We thought that the pain would be due to changes in posture, so we did not pay particular attention to it.2154_A Figure 1. Colonoscopic findings. Approximately 15mm lateral spreading tumor is located on the lower-ascending colon.Four hours later, the patient developed a fever of 37?, complained about exacerbation of the right cervical pain, and started to complain of right-sided abdominal pain. There was tenderness on the right flank to the back, and on the right neck, with no definite signs of peritoneal irritation. Clear crepitus was palpated around the neck, the right side of the chest, and the right back. A computed tomography (CT) scan revealed massive pneumoretroperitoneum around the dorsal side of the ascending colon to the back side of the lumen, pneumomediastinum, and extensive subcutaneous emphysema around neck, right side of the chest, and right flank to the back of the abdomen, consistent with palpation (Figure 2). We decided on conservative treatment because his condition was stable and there were no signs of peritonitis. Under conservative treatment, his symptoms gradually improved without the need for surgical intervention.2154_B Figure 2. Computed tomography (CT) findings. (a) Subcutaneous emphysema of the neck. (b) Pneumomediastinum. (c) Right pneumoretroperitoneum and subcutaneous emphysema at right renal level. (d) Pneumoretroperitoneum around the dorsal side of the ascending colon just with clips after EMR back side of the lumen.DISCUSSION: The majority of the signs of colorectal perforation are abdominal symptoms, such as acute peritonitis. However, in this case, the patient complained of right cervical pain after EMR was performed. It seemed to penetrate the retroperitoneum and was the only initial symptom. Initially we had no doubts about perforation. It seems that the penetrating air reached to the neck in a very short time. Perforation by colonoscopy usually causes symptoms of peritonitis due to intraperitoneal perforation, but in the case of retroperitoneal penetration, emphysema develops outside the peritoneum, causing various symptoms located in parts other than the abdomen, such as the neck. Since cervical pain alone can manifest as a sign of perforation, we should try to find the perforation earlier based on a few symptoms.