EightyThe relative importance of corneal and scleral rigidity and of corneal thinning in the measurement of intraocular pressure is not clearly This study was planned to investigate a group of patients with keratoconus to determine which of these previously reported findings could be substantiated and to identify other possible factors which could be important in the routine clinical measurement of intraocular pressure in patients with low corneal rigidity or corneal thinning. MATERIALSEighty-five patients with keratoconus and 20 normal controls were concurrently investigated. The normal controls were emmetropic, with normal corneal thickness (0.53 k 0.03 mm),' normal corneal curvature (mean radius of curvature 7.7 k 0.2 mm, corneal astigmatism 0.4 k 0.3 di~ptres),~ normal intraocular pressure (13 k 2 mmHg)* and normal axial length of the eye (23.57 k0.91 mm).9The mean age of the 12 male and eight female control subjects was 36 f 17 years (range 14-67) Reprint requests: Anne M. V. Brooks,