The ratio of the diameter of the optic cup to that of the optic disc was studied in three independent samples of first degree relatives, one of which included those of glaucoma patients. The frequency distribution of this ratio did not differ for the three samples and was not influenced by age or sex. The three studies showed independently that in the individual subject, the ratio was similar for the two eyes and varied directly and significantly with that of his parents and with that of his siblings. It is proposed that this ratio is genetically determined and controlled by multifactorial inheritance.