A 59-year-old man presented with Terrien's marginal degeneration associated with keratoconus of his left eye. Penetrating keratoplasty of the left eye permitted an evaluation under optical microscopy, as well as transmission and scanning electron microscopy. An anatomical-pathological evaluation confirmed the co-existence of a central lesion due to keratoconus and a peripheral lesion due to Terrien's marginal degeneration. An absence of the knowledge of the etiology of either affliction does not allow one to define whether there exists one or two active degenerative processes.