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 V_i was 0.974cm³ in the control, whereas it was only 0.596 and 0.255 cm³ in the maturityonset and growth-onset diabetic groups, respectively. The hypertensive group gave almost the same value as the control. There was an obvious negative correlation between V_i and the maximum blood sugar level during glucose tolerance test, whether the case was diabetic or not. Moreover, in the diabetic group V_i diminished with descending age of onset. These findings indicate the importance of V_i 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 N_i decreased with increasing mean radius_??_, and the diabetic and control cases shared a common regression of N_i on _??_. The diabetic pancreas was not characterized by N_i, _??_ or by the distribution pattern of r.