Objective: To determine whether there is a difference in disease severity between familial and sporadic primary open-angle glaucoma (POAG).Methods: A cross-sectional study design compared the distribution of Glaucoma Inheritance Study in Tasmania (GIST) severity scores of patients with genealogically confirmed familial POAG and those with sporadic POAG. The GIST severity scores provide a combined weighting of glaucoma severity based on findings from visual field defects and optic disc analysis, with and without intraocular pressure. A Poisson regression analysis, t test, and chi(2) tests were performed.Results: One thousand twelve (59.5%) of 1700 subjects had familial glaucoma. The mean +/- SD age at examination was greater in the sporadic POAG group compared with the familial group (72.6 +/- 10.3 years vs 70.6 +/- 12.6 years; P=.001). The family group was significantly younger at diagnosis than the sporadic group (mean +/- SD, 61.4 +/- 13.0 years vs 64.0 +/- 12.6 years; P <.001). The GIST severity scores were significantly skewed toward greater disease severity in the familial group compared with the sporadic group (P <.001).Conclusion: Identifying individuals at risk of severe POAG will be more successful if screening programs are developed with appropriate weighting toward those with a positive family history of the disease.