Intraductal papillary mucinous tumors (IPMTs) of the pancreas are rare tumors characterized by a malignant potential. Because of the progress of imaging procedures, smaller cystic pancreatic lesions are now detected and some of them correspond to IPMTs that involve ectatic pancreatic branch ducts but spare the main pancreatic duct. To investigate differences in morphology and clinical behavior of branch and main duct types of IPMT, a surgical series of 43 cases was studied. All pathologic specimens of IPMT, surgically resected in our institution between October 1987 and July 1998, were analyzed. In all cases, the entire pancreatic specimen was systematically examined. IPMT of the branch type was found in 13 (30%) patients, whereas IPMT of main pancreatic duct type that involved the main pancreatic duct and branch ducts was observed in 30 (70%) patients. Patients with IPMT of the branch type were younger (median age, 55 yrs vs 64 yrs), and all but one of the lesions were located in the head and neck of the pancreas (vs 17 of 30 patients with the main duct type). The size of the cysts ranged from 4 to 55 mm, and the major duct showed a mild dilation in most cases. In contrast to the main pancreatic duct type, which showed invasive carcinoma and in situ carcinoma in 11 (37%) of 30 patients and 6 (20%) of 30 patients, respectively, IPMT of the branch type showed significantly less aggressive histologic lesions with five (39%) patients with simple hyperplasia, six (46%) patients with atypical hyperplasia, and two (15%) patients with in situ carcinoma. No invasive carcinoma was observed in this group. IPMT of the branch type occurs in younger patients and is associated with less aggressive histologic features than is the main pancreatic duct type. Our findings raise the difficult issue of clinical management of IPMT of the branch type as a distinctive group.