The study of histological sections of 406 cases of nonendocrine pancreas carcinoma at Memorial Hospital indicated that morphological patterns of pancreas carcinoma could be delineated as follows: duct cell adenocarcinoma (76%), giant-cell carcinoma (5%), microadenocarcinoma (4%), adenosquamous cancinoma (4%), mucinous adenocarcinoma (2%), anaplastic carcinoma (2%), cystadenocarcinoma (1%), acinar cell carcinoma (1%), carcinoma in childhood (under 1%), unclassified (7%). In 195 cases of patients with pancreas carcinoma, search was made for changes in the pancreas duct epithelium and these were compared to duct epithelium in a control group of 100 pancreases from autopsies of patients with nonpancreatic cancer. The following incidences were found for pancreas cancer and nonpancreatic cancer, respectively: mucous cell hypertrophy, 39 versus 28%; pyloric gland metaplasia, 28 and 17%; epidermoid metaplasia, 6 and 12%; papillary hyperplasia, 42 and 12%; atypical duct hyperplasia, 14% and none; cancinoma in situ in 19% and none in the control group. Mucin in the majority of pancreas cancers suggested that the cell type of origin of the common pancreas cancer is the mucin-producing duct epithelium. The association of atypias and carcinomas in situ in the patients with pancreas carcinoma implies, by analogy to other organs, that there may be a significant latent period between the appearance of carcinoma in situ and the grossly recognizable pancreas cancer.