This study examines the prevalence and clinicopathological features of posterior hyaloid membrane separation, or 'true' vitreous detachment, in a randomly selected population of general medical in-patients. One hundred and seven eyes of 60 patients were examined clinically using slit lamp biomicroscopy; in 57% the posterior hyaloid membrane was detached and in 43% the posterior hyaloid membrane was attached. Eighteen eyes have subsequently been examined immunohistochemically, demonstrating that the posterior hyaloid membrane, including the Weiss ring, is made up of type IV collagen. The implications of these findings are discussed.