In 204 adult, white diabetics the palpebral fissure was measured and related to the state of metabolic control of the diabetes. A control group was formed of 204 persons matched by age and sex. The average palpebral fissure in the control group was 9.9 mm: there was a slight, but not significant, difference between the age groups and the sexes. In diabetics who were not insulin dependent the average width of the palpebral fissure was found to be 9.4 mm; in severe chronic deregulation of the diabetes, however, an average palpebral fissure of 8.0 mm was found, a significant narrowing. In insulin-dependent diabetics the average width of the palpebral fissure was 8.3 mm. This significant narrowing also increased if there was severe chronic deregulation of the diabetes. The average palpebral fissure associated with proliferative retinopathy in insulin-dependent type 1 diabetics was 6.0 mm (nearly 4 X standard deviation). This ptosis in diabetes is very probably due to chronic tissue hypoxia, to which the levator palpebrae muscle is probably extra sensitive, and in which thickening of the basal membrane of the capillaries may be one of the most important factors.