Introduction: Duodenal diverticula are most commonly found in the second part of the duodenum adjacent to the ampulla of Vater. Periampullary diverticula (PAD) are extraluminal pseudo-diverticula lacking a mucosal layer. Although typically asymptomatic, they can rarely cause pancreaticobiliary obstruction. Obstructive jaundice can develop secondary to PAD without choledocholithiasis, a rare phenomenon called Lemmel’s Syndrome. We report a rare case of Lemmel’s syndrome presenting not only with acute cholangitis but also with concurrent pancreatitis, successfully treated with ERCP. Case Description/Methods: An 83-year-old female presented with upper abdominal pain, nausea, diarrhea, and rigors. She is s/p cholecystectomy and labs revealed AST 688 U/L, ALT 305 U/L, Alkaline Phosphatase 201 U/L, total bilirubin 3.4 mg/dL, lipase 968 U/L, and lactate 2.9 mmol/L. An abdominal CT scan showed severe dilation of the common bile duct of 2.1 cm with edematous appearance of the pancreatic head, findings of acute pancreatitis, and a large duodenal diverticulum adjacent to the pancreatic head (Fig 1). Her hospital course was complicated by gram negative bacteremia, concerning for acute cholangitis. MRCP confirmed marked dilation of the common bile duct extending to the Ampulla of Vater with a large periampullary diverticulum, without choledocholithiasis or mass lesions (Fig 2). The patient underwent ERCP which revealed a large duodenal diverticulum compressing the distal bile duct, treated with biliary sphincterotomy and stent placement (Fig 3-4). She showed gradual clinical improvement with this endoscopic approach and was discharged home with subsequent follow up for stent removal performed 6 weeks later. Discussion: Although common among duodenal diverticula, PAD are rarely symptomatic. The underlying pathogenesis remains unclear but is believed to be related to mechanical obstruction, inflammation, and fibrosis. Lemmel’s syndrome can be mistaken for pseudocysts, necrotic collections, periampullary tumors, and pancreatic head neoplasms. Physicians should have a high index of suspicion for Lemmel’s syndrome in patients with large symptomatic PAD, which although commonly presents as biliary obstruction causing acute cholangitis, can rarely also cause pancreatic obstruction leading to pancreatitis, as seen in our patient. Timely diagnosis and endoscopic treatment of Lemmel’s syndrome can be effective in the treatment of acute cholangitis and pancreatitis.Figure 1.: CT scan showing CBD dilation with large periampullary diverticulum.Figure 2.: MRI showing dilated CBD and PD with distal large periampullary diverticulum.Figure 3.: Endoscopic view of large periampullary diverticula with intradiverticular papilla seen on ERCP.Figure 4.: Fluoroscopic view of dilated CBD as a result of Lemmel’s syndrome.