Two monozygotic twin pairs with primary open-angle glaucoma (POAG) derived from the Finnish Twin Cohort Study were studied. The first female twin pair was concordant, and the second female pair was discordant for primary open-angle glaucoma. The nondiseased member of this second pair showed preglaucomatous signs in the form of ocular hypertension. The clinical study of the patients included a careful ophthalmologic examination, retinal nerve fiber layer (RNFL) photography and optic disc stereophotography. Visual fields were tested with an automated perimeter (Octopus 2000). The small number of twin pairs in the present study does not permit quantitative analysis of heritability, but it is obvious that whenever a clinical diagnosis of primary open-angle glaucoma in a monozygotic twin is made, the co-twin should be carefully examined.