The sella turcica volumes of normal children, children with idiopathic hypopituitarism and children dwarfed from other causes, have been compared to determine whether sellar volume is a function of body size or of chronological age. These data were used to assess the value of such measurements in the diagnosis of pituitary dwarfism. The mean volume for the non-hypopituitary dwarfs was less than that of the normal children when the groups were compared by age. When compared by height, however, the group means were not significantly different. Thus, linear growth failure from causes other than growth hormone deficiency may result in proportionate retardation in sellar growth. The mean volume of the growth hormone deficient group was significantly less than either the normal children or the non-hypopituitary control dwarfs, when compared by both age and height. These findings suggest that the pituitary gland of many patients with idiopathic hypopituitarism is abnormally small. The sella volumes of 1/3 of the hypopituitary dwarfs were more than 2 sd below the age and height related means of the control group. Although the finding of an abnormally small sellar volume is highly suggestive of hypopituitary dwarfism, a normal sellar volume does not exclude the diagnosis.