Quantitative changes of the pancreatic islets in diabetes mellitus were analyzed by a stereological method. 26 maturity-onset and 5 growth-onset diabetics, and 37 nondiabetics including 9 hypertensives were selected from autopsy materials and the pancreases were subjected to histometry. The total islet volume Vi was 0.974 cm3 in the control, whereas it was only 0.596 and 0.255 cm3 in the maturity-onset and growth-onset diabetic groups, respectively. The hypertensive group gave almost the same value as the control. There was an obvious negative correlation between Vi and the maximum blood sugar level during glucose tolerance test, whether the case was diabetic or not. Moreover, in the diabetic group Vi diminished with descending age of onset. These findings indicate the importance of VI in the pathophysiology of diabetes and support the classical concept of insulin deficiency as the primary pathogenetic role. On the other hand, the total islet number Ni decreased with increasing mean radius r, and the diabetic and control cases shared a common regression of Ni on r. The diabetic pancreas was not characterized by Ni, r or by the distribution pattern of r.