A 15-year-old boy was admitted to our hospital with a recent increase in the size of a preexisting pancreatic pseudocyst. At 11 years of age, he was diagnosed with acute lymphoid leukemia (ALL) and received chemotherapy with L-asparaginase (L-Asp); he developed the pancreatic pseudocyst following L-Asp-induced acute pancreatitis. The pancreatic pseudocyst had increased to 120mm in diameter. He developed epigastralgia and portal hypertension. Endoscopic ultrasound (EUS)-guided cystogastrostomy with the placement of a 7-cm 7-Fr plastic stent and a 5-Fr NB pigtail catheter led to the near-complete resolution of the pseudocyst. There were no signs of recurrence within the first year after intervention. EUS-guided drainage, increasingly used for pseudocysts, should be considered as an effective treatment approach for pediatric pancreatic pseudocysts.