We evaluated the occurrence or progression of nuclear sclerosis of the crystalline lens in 100 eyes after vitrectomy for removal of idiopathic epiretinal membranes causing macular pucker. The follow-up period ranged from six to 99 months (average, 29 months). Visually significant nuclear sclerosis was present preoperatively in three of the operated on eyes and four of the fellow eyes. The rate of occurrence or progression of visually significant nuclear sclerosis was far greater in the operated on eyes (P less than .0001). Of 100 patients, 80 operated on eyes and 24 fellow eyes had visually significant nuclear sclerosis or had undergone previous cataract extraction at the conclusion of the study. Different concentrations of glucose in the intraocular irrigating solution did not affect occurrence of later nuclear sclerosis. Patients older than 50 years of age had a far greater incidence of later nuclear sclerosis than patients younger than 50 years of age (P = .0003). Nuclear sclerosis may be caused by altered lens metabolism after removal of part of the vitreous gel, since nuclear sclerosis also occurs in other conditions associated with vitreous liquefaction.