The search for environmental factors associated with cataract has produced conflicting evidence and one possible reason may be that environmental influences are initiating events occurring perhaps for short periods many years before loss of sight from cataract. It is important, therefore, to be able to detect the earliest changes of cataract formation so that epidemiological studies have a better chance of detecting environmental factors. Two possible indicators of early cataract are delayed growth of the lens and abnormalities of the anterior subcapsular clear zone of the lens as observed on slit-lamp microscopy. A series of patients with early lens changes was compared with control subjects in respect to these two factors. Lens thickness was measured by a simple optical method. The mean thicknesses of the lens in patients with early cortical or posterior subcapsular lens changes were significantly less than that of age matched controls. 60% of lenses with early cataract of all types were found to have a deficient or absent anterior subcapsular clear zone. Lens thickness and the appearance of the anterior subcapsular clear zone are easy to measure and observe through an undilated pupil. Although the prognostic value of the results is uncertain in individual cases owing to the rather wide scatter of results in normal eyes, such observations could be of value in comparative studies of populations.