['To compare the prevalence of angle closure among siblings of patients with open angles (OAs), suspect angle closure (PACS), and either primary angle closure (PAC) or PAC glaucoma (PACG).', 'Cross-sectional, clinical study.', 'A total of 303 South Indian sibling pairs, including 81 OA probands, 143 PACS probands, and 79 PAC/PACG probands.', 'Probands and siblings underwent a clinical examination, including gonioscopy by a masked grader, applanation tonometry, slit-lamp biomicroscopy, optic nerve evaluation, and A-scan ultrasonography. Probands and siblings were classified into 1 of 3 groups based on the phenotype of the more severely affected eye: OA, PACS, or PAC/PACG. Multivariable regression models were used to estimate the odds of prevalent angle closure in PACS or PAC/PACG siblings compared with OA siblings.', 'Prevalence and relative prevalence of angle closure and PAC/PACG among OA, PACS, and PAC/PACG siblings.', 'Mean sibling age was 49.7 ± 8.7 years, and 56.6% of siblings were females. Angle closure was more prevalent in both PACS siblings (35.0%) and PAC/PACG siblings (36.7%) compared with OA siblings (3.7%; P < 0.001). There was PAC/PACG present in 11.4% of PAC/PACG siblings compared with 4.9% of PACS siblings (P = 0.07) and 0% of OA siblings (P = 0.002). In multivariable models adjusting for sibling age and sex, the odds of angle closure was 13.6 times greater in angle closure (PACS or PAC/PACG) siblings compared with OA siblings (95% confidence interval [CI], 4.1-45.0; P < 0.001). Sibling angle-closure risk was also greater in female (odds ratio [OR], 2.3; 95% CI, 1.3-4.0; P = 0.005) and older siblings (OR, 1.5 per 10-year increment; 95% CI, 1.1-2.0; P = 0.02). Siblings of PAC/PACG probands had a 2.3-fold greater odds (95% CI, 0.8-6.5) of having PAC/PACG compared with siblings of PACS probands, although the association was not significant (P = 0.13).'] Is family history a strong risk factor for prevalent angle closure in a South Indian population?