A study was undertaken to determine the prevalence of chronic simple glaucoma among blood relatives of persons blind from glaucoma, the occurrence of other ocular diseases among these relatives, and the effectiveness of a single tonometer reading in detecting glaucoma. Forty-three persons blind in one or both eyes yielded 173 blood relatives, of whom 136 participated in the study. Nine of these 136 persons, or 6.6%, had glaucoma or borderline glaucoma, three times the expected yield among the general population. One or more ocular diseases (excluding refractive errors) occurred in 53, or just less than two out of five relatives, but when glaucoma or suspect glaucoma was also excluded, only about one fourth had other ocular diseases. The effectiveness of a single tonometer reading could not be determined because the cases were not newly identified in the study.