Intraductal papillary-mucinous tumors (IPMTs) of the pancreas are characterized by dilated pancreatic ducts and ductules that are lined by tall columnar mucin-producing neoplastic epithelial cells. IPMTs have been suggested to be distinct neoplasms with a less aggressive phenotype than that of conventional ductal adenocarcinomas of the pancreas. Molecular mechanisms underlying tumorigenesis of IPMTs are beginning to be characterized. Allelic losses have frequently been detected at 9p, 17p, and 18q, suggesting that inactivation of tumor suppressor genes at these loci play a role in tumorigenesis of IPMTs. Using immunohistochemistry, we analyzed 38 IPMTs, including 9 hyperplasia, 16 adenoma, and 13 carcinoma tissues, for expression of p53, Ki-67, p16, and SMAD4. Nuclear p53 expression was observed in 5 (38%) of 13 carcinoma tissues but not in hyperplasia or adenoma tissues. Partial loss of p16 expression was observed in 9 (56%) and 12 (92%) of the 16 adenoma and 13 carcinoma tissues, respectively. Partial loss of p16 expression was observed even in adenomas with mild atypia. Partial loss of SMAD4 expression was observed in 6 (38%) and 12 (92%) of the 16 adenoma and 13 carcinoma tissues, respectively. The SMAD4 negative index was significantly higher in invasive carcinomas than in non-invasive carcinomas. Our results suggest that loss of p16 is an early event and p53 alteration is a late event during the progression of IPMTs. SMAD4 inactivation appears to be an early event but also involved in invasive tumor growth. Our results suggest that these alterations accumulate during the progression of IPMTs, reflecting results of analysis of allelic losses that showed a stepwise accumulation of genetic changes during progression.