Ninety-nine eyes with keratoconus of 64 patients were examined using the sagittal radii method to determine the corneal flattening and compared with 100 eyes of 50 healthy controls. To express the degree of this flattening, four numerical eccentricity (E) values for each eye were calculated. The highest E value of the four was chosen as a parameter for diagnosis of the disease. In keratoconus patients in whom the highest E values of keratoconus eyes were 3=0.8, the specificity of the test was 98% and the sensitivity was 97%. This abnormal eccentricity was observed in keratoconus eyes with a visual acuity of 1.0, whereas slit-lamp examination showed no abnormalities. This study demonstrates that eccentricity can be helpful to diagnose keratoconus at an early stage.