BACKGROUND The aim of the study was to evaluate whether, in primary open-angle glaucoma (POAG), patients younger than 40 years differ in optic disc morphology from patients older than 40 years. METHODS Out of a total group of 419 patients with POAG, we formed and compared two subgroups, one consisting of 37 patients with an age of less than 40 years, the other composed of 382 patients with an age equal to or more than 40 years. Both subgroups were matched for neuroretinal rim area. We examined the optic disc morphometrically using stereo disc photographs. RESULTS The younger subgroup, as compared to the older subgroup, showed deeper and steeper optic disc cupping, concentric emaciation of the neuroretinal rim, a significantly smaller area of parapapillary atrophy, and significantly higher maximal and minimal intraocular pressure measurements (P < 0.001). The size and shape of the optic disc and the diameter of the retinal vessels at the optic disc border did not vary significantly. CONCLUSIONS In POAG, patients younger than 40 years differ in optic disc morphology from patients older than 40 years. The younger patients with POAG have high minimal and maximal intraocular pressure readings and an optic disc morphology with deep and steep cupping, concentric loss of neuroretinal rim, and an almost unremarkable parapapillary atrophy. POAG in patients under 40 represents chronic high-pressure open-angle glaucoma with mainly diffuse optic nerve damage.