Raman spectra were measured for cac-strain mouse (Nakano mouse) lenses in the various stages of cataract formation. The spectra were compared with those of normal mouse lenses of the corresponding ages. A significant difference was observed in the intensity of the Raman band due to an OH stretching mode of lens water (3390 cirri) between the spectra of cataractous lenses and those of normal lenses. The difference was already obvious in the very incipient stage of hereditary cataract and became more pronounced with cataract development. These observations clearly show that changes in lens water occur during cataractogenesis. We propose in this communication that the intensity change of the Raman band at 3390 cm−1may beveryuseful for the diagnostic marker of cataractous signatures.