A nationwide record linkage of the Finnish Twin Cohort Study (FTCS) with the Hospital Discharge Registry and the Registry of Rights for Free medication is presented. This study consists of 108 pairs (114 individuals) of twins with chronic open-angle glaucoma. Of the twin pairs 29 were monozygotic (MZ), while 79 pairs were dizygotic (DZ). Three monozygotic and 3 dizygotic pairs were concordant for chronic open-angle glaucoma (OAG), while 26 monozygotic and 76 dizygotic pairs were discordant. Seventy-three twins had chronic simple glaucoma, while 34 twins had capsular glaucoma, and in 7 patients chronic simple glaucoma was noted in one eye and capsular glaucoma in the second eye. The heritability of chronic open angle glaucoma was 13%. The higher O/E-ratio of concordant twin pairs among MZ twins indicates that genetic factors play a role in this disease. The difference is anyhow surprisingly low compared to former estimates of heritability of open angle glaucoma. The steep rise in prevalence in older age groups was confirmed. The age-adjusted prevalence of chronic open-angle glaucoma in this study was 0.63%.