A simple system has been developed for the clinical grading of the presence and severity of lens opacities. The densities of nuclear opacities as seen on clinical slit-lamp examination are graded in comparison with a set of standard photographs. The extent of cortical opacities seen on retroillumination is estimated in terms of segments involved. The dimensions of posterior subcapsular opacities are measured using the slit beam. Inter- and intraobserver trials were conducted with three observers. Twenty subjects were selected to provide a range of lens opacities. Weighted Kappa statistics for intraobserver agreement for these three types of opacities range between 0.58 and 1.00 and for interobserver agreement between 0.41 and 0.71. This clinical grading system shows good reproducibility and compares well with the grading of lens opacities in photographs. It should prove to be of considerable use in clinical and epidemiological studies of cataract.