Cross-sectional studies and those using national data sets estimate that glaucoma-related blindness is between six and eight times more common among black Americans than among whites. Community-based studies have found that glaucoma is four to six times more prevalent among blacks. It is not known why blacks with glaucoma are more likely to become blind than whites with glaucoma. To investigate the possibility that under-treatment of glaucoma is an important factor contributing to this higher rate of blindness, we studied the population-based rates of incisional and laser surgery for open-angle glaucoma among blacks and whites in a 5 percent random sample of Medicare claims for 1986 through 1988. For all U.S. census divisions combined, the rate of surgery for glaucoma among black Medicare beneficiaries was 2.2 times higher than the rate among white beneficiaries (95 percent confidence interval, 2.1 to 2.3). We calculated an expected rate of treatment among blacks on the basis of the rate of treatment among whites and the assumption that glaucoma is four times more prevalent among blacks — a conservative estimate. The observed rate of glaucoma surgery among blacks was 45 percent lower than the expected rate we calculated, which may in part account for the excess rate of blindness among blacks. The magnitude of this difference in treatment rates varied from 29 percent in the Middle Atlantic states to 50 percent in the South Atlantic states. Older black Americans are not receiving potentially sight-saving care for open-angle glaucoma at the same rate as older white Americans.