Evidence is presented that idiopathic senile macular hole is caused by focal shrinkage of the vitreous cortex in the foveal area. The most reliable biomicroscopic signs of impending hole formation (stage 1) are the development of a yellow spot or ring in the center of the fovea, loss of the foveal depression, and no evidence of separation of the vitreous from the foveal retina. Although in a majority of eyes with stage 1 changes there is a progression to hole formation, spontaneous separation of the vitreous without hole formation may occur in some cases (44%) and cause characteristic biomicroscopic changes, including foveal reattachment, disappearance of the yellow spot or ring, and, in some cases, a pseudo-operculum, with one or more lamellar holes or facets. A prospective collaborative study is recommended to confirm these findings and to test the clinical value of surgical peeling of the vitreous cortex in eyes with stage 1 changes as a means of preventing hole formation.