Treatment of "forme fruste" choledochal cyst (FFCC) where pancreaticobiliary malunion (PBMU) is associated with minimal dilatation of the common bile duct (CBD) remains controversial. PBMU allows pancreaticobiliary reflux to occur, which causes complications such as recurrent pancreatitis and gallbladder cancer. Therefore, some surgeons opt to treat FFCC by cholecystectomy alone in order to prevent gallbladder cancer, with the result that pancreaticobiliary reflux could still occur. Our treatment of choice-excision of both the CBD and the gallbladder, followed by Roux-en-Y hepatico-jejunostomy-can eliminate pancreaticobiliary reflux and prevent complications. Our case, a 2-year-old boy, initially presented to a hospital abroad with recurrent abdominal pain. Endoscopic retrograde cholangiopancreatography showed massive protein plugs impacted in the papilla of Vater and mild CBD dilatation, but PBMU was not identified. Intraoperative cholangiography performed during laparotomy 5 days later suggested PBMU with minimal CBD dilatation. Despite these findings, cholecystectomy with T-tube drainage was performed rather than CBD excision with biliary reconstruction. Postoperative T-tube cholangiography clearly showed PBMU. The T-tube was removed after 2 weeks, and 3 months later the boy was referred to us because of recurrent pancreatitis. We performed CBD excision and Roux-en-Y hepatico-jejunostomy. His postoperative course was uneventful, and he is well after 10 years of follow-up. This case provides strong evidence that CBD excision with biliary reconstruction is mandatory for treating FFCC and, conversely, that cholecystectomy alone is inadequate for treating children with FFCC.