Most intraductal papillary-mucinous carcinomas (IPMCs) of the pancreas are resectable and curable, but some develop into frankly invasive carcinomas. We studied the clinicopathologic features of eight cases of invasive carcinoma derived from IPMC (IC-IPMC) of the pancreas. The patients were aged 54-75 years (mean, 66.6 years); six were male and two were female. The mean tumor size was 7.7 cm (range 5.5-10.5 cm). Two patients without lymph node metastasis had no peripancreatic invasion, and survived longer (115 and 20 months). Three out of four patients with extrapancreatic invasion died of their tumors or developed tumor recurrence within a year. One patient with evidence of liver and lymph node metastasis at the time of first surgery again showed metastatic tumor 21 months later. One patient died of another cause. We also performed a comparative study of the immunohistochemical features of IC-IPMCs in 9 IPMCs (including minimally invasive cases) and 15 ductal adenocarcinomas. CEA cytoplasmic positivity was observed in most of the IC-IPMCs (87.5%) and ductal adenocarcinomas (93.3%), but in only 1 IPMC (11.1%). The frequency of p53 nuclear staining in ductal adenocarcinoma (73.3%) was higher than in IPMC (33.3%) or IC-IPMC (37.5%). In conclusion, IC-IPMC with extrapancreatic invasion should be treated as ductal carcinoma because of its aggressive behavior after resection. Some IPMCs might progress to invasive carcinoma via pathways that are different from those followed by ductal adenocarcinomas.