Purpose: To investigate the relationship between the defect in the premacular cortical vitreous detached from the retina and the development of idiopathic premacular fibrosis. Methods: One hundred six consecutive patients (106 eyes) with posterior vitreous detachment but no other ocular disorders were prospectively studied. All follow-up periods were longer than 6 months. The vitreous condition was studied using an aspherical, +58.6-diopter preset lens with a slit-lamp. Results: The detached premacular cortical vitreous was a continuous, unbroken sheet in 93 (88%) of the 106 eyes; in 13 (12%) eyes, the detached premacular cortical vitreous had an oval or round defect ranging from 4 to 7 disc diameters over the macula. At the study's conclusion, 36 (34%) of the 106 eyes had idiopathic premacular fibrosis; 70 (66%) eyes did not. The defect in the detached premacular cortical vitreous was observed in 9 (25%) of the 36 eyes that eventually developed idiopathic premacular fibrosis and in 4 (6%) of the 70 eyes that did not undergo such development. The incidence of the defect in the detached premacular cortical vitreous was significantly higher in eyes with idiopathic premacular fibrosis than in eyes without (P = 0.011). Nine (69%) of thirteen eyes with the defect in the detached premacular cortical vitreous developed premacular fibrosis, whereas only 25% of eyes without the defect developed it. Twenty-seven (75%) of thirty-six eyes with premacular fibrosis did not have a defect in the detached premacular cortical vitreous. Conclusions: The defect in the detached premacular cortical vitreous probably occurs because the premacular cortical vitreous remains attached to the internal limiting lamina when a posterior vitreous detachment develops. The vitreous that remains attached to the macula may be related to the development of idiopathic premacular fibrosis.