Lens examinations were performed on 7 obligate and 7 possible carriers of the X-linked gene for Lowe's syndrome, and on 117 controls. By quantitatively grading punctate cortical opacities, it was possible to discriminate between the obligate carriers and the controls with a fair degree of confidence. In the age group most important for genetic counselling, that of child bearing, the data are too limited for the derivation of precise estimates, but may, nevertheless, be useful. More such data are needed.