The pathological features of 12 acinar cell neoplasms of the pancreas are described; these comprise 11 carcinomas, of which seven were pure acinar cell growths and four were mixed acinar and ductal carcinomas, and one adenoma. These tumors occurred in a series of 105 during the period 162-75. Thrombotic endocarditis developed in three out the 11 carcinoma cases. The distinctive histological features of these neoplasms and the means of differentiating them from anaplastic carcinomas and certain other carcinomas, for example, islet cell carcinoma, oat cell carcinomas, and carcinoid tumours, are discussed. The poor prognosis of pancreatic cancers is emphasized, and reasons are put forward for believing that future epidemiological studies may need to take account of the histological types of pancreatic carcinoma.