We report 2 cases of pancreatic endocrine tumors that caused obstruction of the main pancreatic duct(MPD).A 49-year-old asymptomatic man was referred to our institution because dilation of the MPD was revealed by abdominal ultrasonography(US).No tumor was detected by endoscopic ultrasonography,computed tomography(CT),and magnetic resonance imaging(MRI).The diameter of the MPD was > 20 mm at the body,and no dilation was noted at the head.Although malignancy was not confirmed through cytology or imaging,pancreatic cancer was strongly suspected.Pancreaticoduo-denectomy was performed.Pathological and immunohistochemical examination revealed a 5 mm × 3 mm serotonin-positive endocrine tumor.Fibrosis was present around the MPD and seemed to cause stricture.A 32-year-old asymptomatic man had elevated serum amylase,and US demonstrated dilation of the MPD.No tumor was detected by CT and MRI.Pancreatic cancer was suspected due to stricture and dilation of the MPD.Pancreatectomy of middle part of pancreas was performed.Pathological and immunohistochemical examination revealed a serotonin-positive endocrine tumor sized 5 mm × 4 mm.We report 2 cases of serotonin-positive pancreatic endocrine tumors that caused stricture of the MPD in spite of the small size of the tumor. We report 2 cases of pancreatic endocrine tumors that caused obstruction of the main pancreatic duct(MPD).A 49-year-old asymptomatic man was referred to our institution because dilation of the MPD was revealed by abdominal ultrasonography(US).No tumor was Detected by endoscopic ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI).The diameter of the MPD was > 20 mm at the body, and no dilation was noted at the head.Although malignancy was not confirmed through cytology or imaging , pancreatic cancer was strongly suspected.Pancreaticoduo-denectomy was performed.Pathological and immunohistochemical examination revealed a 5 mm × 3 mm serotonin-positive endocrine tumor.Fibrosis was present around the MPD and seemed to cause stricture.A 32-year-old asymptomatic man Had elevated serum amylase, and US demonstrated dilation of the MPD.No tumor was detected by CT and MRI.Pancreatic cancer was suspected due to stricture and dilation of the MPD.Pancreatectomy of middle part of Pancreas was performed. Pathological and immunohistochemical examination revealed a serotonin-positive endocrine tumor sized 5 mm × 4 mm.We report 2 cases of serotonin-positive pancreatic endocrine tumors that caused stricture of the MPD in spite of the small size of the tumor.