Pancreas from 10 children with idiopathic hyperinsulinemic hypoglycemia was examined using histochemical and immunostaining techniques. The children ranged from newborn to 9 months in age. Sections were studied with particular reference to islet cell distribution in patients and controls, and quantitative assessments were made of islet size, relative cell-type distribution, and total area of pancreas occupied by endocrine tissue. Four had islet cell adenomatosis, three of these focal and one generalized. The others had a subtle morphologic abnormality seen best on immunostained sections and characterized by loss of the usual centrilobular congregation, irregular islet contours, a generalized of small packets of endocrine cells throughout the acinar tissue, and islet cell hypertrophy. We have termed this constellation "endocrine cell dysplasia." The range of islet cell area found in the controls using immunostaining was substantially higher than previously reported. In addition, we found no increase in mean total endocrine area in the cases with endocrine dysplasia when compared to age-matched controls. Both classic and beta-cell nesidioblastosis were common to patients and controls alike, appeared to decrease with age, and thus could not be considered as the morphologic substrate of hyperinsulinism in this age group.