The percentile volumes vA, vB and vD of A, B and D cells in the islets of Langerhans were histometrically estimated in the pancreases from 59 autopsy cases including maturity- and growth-onset diabetics and nondiabetics. Volumetry was performed microscopically by point counting method using a square-lattice eyepiece, then the total A, B and D cell volumes VA, VB, and VD were calculated from the total islet volume Vi of the pancreas in VB between the three groups. Namely, it was only 0.156 and 0.388 cm3 in the growth- and maturity-onset diabetics, much smaller values than 0.636 cm3 in nondiabetics. The simultaneous investigation on clinical records revealed a negative correlation between VB and the maximum blood sugar of the individual. The smaller VB in diabetes was attributed to the diminution of Vi because vA, vB and vD were almost the same regardless of the presence or absence of diabetes. These results strongly suggested a failure of insulin production in diabetes due to quantitative deficiency of B cells.