We present the findings on a 27-year-old male with eosinophilic gastro-enterocolitis accompanied with acute pancreatitis. Acute pancreatitis may be induced by pancreatic duct obstruction caused by marked swelling of the papillary region of the duodenum due to eosinophil infiltration. After prednisolone treatment, clinical manifestations rapidly improved; the serum amylase decreased and the peripheral eosinophilia was recovered. The serum interleukin-5 (IL-5) level was high at diagnosis and decreased by prednisolone therapy; however, IL-5 was detected by enzyme immunoassay even during clinical remission. These results indicate that eosinophilia is mediated by IL-5, and detectable levels of IL-5 indicate the possibility of relapse.