Seven patients with type I diabetes mellitus (Group 1), seven with normoglycemic cystic fibrosis (Group 2), seven with hyperglycemic cystic fibrosis (Group 3), and ten age-matched control subjects underwent corneal fluorophotometry and quantitative specular microscopy. Group 1 had background microangiopathic retinopathy but no evidence of proliferative disease by fluorescein angiography. Significant increases in mean corneal endothelial permeability and mean pump rate occurred in Group 1, indicating a defect in the endothelial barrier function early in type I diabetes mellitus. Similar significant increases in mean corneal endothelial permeability and mean pump rate occurred in both cystic fibrosis groups. The greatest increase was found in Group 3, suggesting a primary defect in the endothelial barrier function in cystic fibrosis, aggravated by the hyperglycemic state. No morphologic abnormalities were noted in Group 1, but both cystic fibrosis groups had smaller mean cell areas than did the control group. There were significant differences in the morphologic and functional correlations between Groups 1 and 3, suggesting different mechanisms for the increased endothelial permeability in these two disorders.