OBJECTIVE To study the incidence and progression of lens opacification after implantation of phakic posterior chamber intraocular lenses for myopia and its correlation with vaulting and endothelial cell density (ECD). METHODS Department of Ophthalmology, University of Vienna Medical School, Vienna, Austria. METHODS An implantable contact lens (ICL V4, Staar Surgical Inc.) was inserted in 76 myopic eyes. Patients were prospectively followed preoperatively and at 1, 3, 6, 12, 24, and 36 months. The uncorrected visual acuity and best corrected visual acuity (BCVA) were determined. Vaulting was measured optically with a Jaeger II pachymeter, and the crystalline lens was examined at the slitlamp for the presence and characteristics of opacification. Endothelial cell morphometry was performed by specular microscopy, and the ECD was calculated. Eyes in which lens opacification developed were followed for at least 12 months to determine the degree and course of visual impairment. RESULTS Lens opacification occurred in 11 eyes (14.5%). Opacification was correlated with intraoperative trauma to the crystalline lens, age older than 50 years, and decreased ECD values throughout the observation period. Vaulting of the ICL did not correlate with the risk for lens opacification. After onset of lens opacification, 6 eyes (55%) had a stable BCVA within +/-0.5 lines and 5 eyes had progressive opacification, losing between 3.5 lines and 0.5 lines (mean 1.8 lines +/- 1.1 [SD]). Three eyes (3.9%) in the progressive group had a 1- to 2-line loss of BCVA over preoperative values and subsequently had cataract surgery. CONCLUSIONS Risk factors for lens opacification after implantation of the model V4 ICL included intraoperative trauma to the crystalline lens and older age. Decreased ECD in eyes with opacification suggests ongoing inflammation as a cause. Patients younger than 45 years may have a significantly lower incidence of opacification.