I have investigated islets of chronic pancreatitis patients histochemically and electron microscopically, and classified chronic pancreatic lesions into six grades according to the degree of fibrosis (Grade 0 to Grade Ⅴ). 1) In Grade Ⅱ, some acinar cells showed irregularly dilated rough endoplasmic reticula, swollen mitochondria in their cytoplasm, and reduction in the number of zymogen granules. In Grade Ⅲ, these alterations were more prominent. 2) As far as islet cells were concerned, before Grade Ⅱ most of the islet cells remained almost normal in structure but some B cells in Grade Ⅱ are characterized by cellular structures that are irregular because of their dilated endoplasmic reticulum. In Grade Ⅲ cells, these alterations were more prominent and mitochondria became rounded in shape and endocrine granules showed irregular formations and a reduction in number. A and D cells, in comparison with B cells, usually exhibited relatively mild alterations. 3) In grade Ⅳ, most of the islet cells were almost wholly occupied by the cisternae of dilated endoplasmic reticulum and cytoplasmic vacuoles; and those few granules that remained had lost almost all their density. Almost all of the A, B and D cells presented these marked alterations, but alterations in the B cells were especially prominent. Even in Grade Ⅳ, since normal nerve endings still existed around the islets and at least one third of the islets still had almost normal granules, the endocrine function of the pancreas seems to be preserved. 4) In Grade Ⅳ, it is of prime interest that in the basal region of ducts islet cells containing almost normal granules were frequently demonstrated. These cells were observed as isolated single cells in the duct at first, most of which were A cells. On the other hand, most of the B cells were located in the clusters. The presence of small islets close to a duct epithelium was also observed frequently. I am convinced that islet cells develop from ductules in human adult pancreas because all these findings are as same as that in human fetus. 5) I observed the other cells than A, B and D cells in human pancreas. They have many granules elongated and irregular in shape. I believe them to be F cells, same as enterochromaffin cells (EC cells) in digestive mucosa.