In sections of the pancreas in 202 cases including 51 diabetics, the islets in each of 10 microscopic fields were drawn by a camera lucida, and their total islet area was estimated by a planimeter. The results, in % of islet to pancreatic tissue, were analyzed to find out if age, sex, or disease were associated with variations in the total islet tissue content, or in the size of number of islets. At the one extreme are cases with low islet tissue content and these are almost exclusively diabetics. At the other extreme are those with a high proportion of islet tissue, and the latter were principally infants, or adults with chronic or proliferative diseases. Under 0.9% of islet tissue is presumably significant of a deficiency of insulin secretion since it occurs in the great majority of cases of diabetes mellitus. From 0.9 to 2.7% may be considered normal for adults, and from 0.9 to 3.6% normal for infants. Over 3.6% is classed as high and probably significant of potential or actual hypoglycemia.