Pancreatic tissue obtained from 26 patients with alcoholic chronic pancreatitis (ACP), nine patients with nonalcoholic idiopathic chronic pancreatitis (NAICP), and seven patients with obstructive chronic pancreatitis (OCP) was studied in an attempt to determine whether clinical or etiologic differences have a morphologic counterpart. Histologically it was easy to distinguish ACP from OCP occurring distal to an obstruction of the pancreatic duct. Nine patients with NAICP showed histological features similar to those found in ACP. Plugs and calcifications were found as frequently in NAICP as in ACP, suggesting that NAICP, whatever the etiology, is truly pancreatolithiasis, which leads to slowly progressive fibrosis and acinar atrophy in the obstructed pancreatic lobule. Nerve fibers were found to be more numerous in all disease categories. Inflammatory foci of lymphocytes associated with nerves were observed in 57 and 35% of cases with OCP and ACP, respectively, but only in one patient with NAICP. These findings may constitute a pathological basis for the existing clinical data showing that NAICP frequently runs a pain-free course.