The percentile volumes υ_A, υ_B and υ_D 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 V_A, V_B and VD were calculated from the total islet volume V_i of the pancreas estimated separately. The measurement disclosed a prominent difference in V_B 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 V_B and the maximum blood sugar level during GTT, showing that V_B reflects grossly the degree of glucose tolerance of the individual. The smaller V_B in diabetes was attributed to the diminution of V_i because υ_A, υ_B and υ_D 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.