From 1983 to 1994 enucleated as well as bulbi obtained post mortem from 20 guinea pigs were ophthalmopathologically examined routinely. Eleven cases (one bilateral) with anamneses as hydrophthalmus, exophthalmus or retrobulbar abscess showed histologically osseous choristomas in the stroma of the ciliary body. In one case the osseous choristoma was already macroscopically recognizable. These heterotopic bone formations were causative for a secondary open angle glaucoma. Furthermore, it explains the obscured clinical symptoms of glaucoma together with the resulting exposition keratopathy. The aetiopathogenesis is discussed comparatively.