ERCP (endoscopic retrograde cholangiopancreatography) represents a safe endoscopic procedure and serious complications (perforation, haemorrhage, and acute pancreatitis) are usually uncommon. We present the case of a 38-year-old patient with gallstones in the common biliary duct who developed acute pancreatitis after ERCP. One month later a huge fluid collection with necrotic tissue in the right paracolic gutter was found, the fluid being drained by percutaneous drainage under ultrasonographic guiding. The particularity of the case is the post-ERCP pancreatitis, complicated with walled-off necrosis, resolved without surgical intervention by using percutaneous drainage.