Phacoemulsification and implantation of a foldable intraocular lens for cataract is almost a routine procedure in ophthalmic practice. Late postoperative opacification of hydrophilic acrylic (hydrogel) intraocular lenses (IOLs) has been reported, but the appearance differs from our findings. We report clinical, pathologic, ultrastructural, and spectroscopic analyses of foldable hydrogel IOLs undergoing late opacification resulting in visual disturbances. This retrospective study comprised 486 patients (550 eyes) who had implantation of a foldable hydrogel posterior IOL. Three cases (three eyes) had been found to have opacification of the IOLs. The three lenses were exchanged because of significant visual disturbances, and were examined using light microscopy, scanning electron microscopy and energy dispersion X-ray (EDX) spectroscopy. Incisional biopsies of the capsular membrane in contact with the opacified IOL were performed in one patient. The mean interval between implantation of the hydrogel IOLs and the diagnosis of opacification was 35.3 months (range, 34-36 months). All three of the cases had quiet anterior chamber. No obvious posterior capsular opacification were found. The best corrected visual acuity of the three cases who had exchanged IOLs were all 20/25 or better. Microscopic analysis revealed multiple, fine, granular structures both on the surface and matrix of the opacified IOLs. EDX analysis showed the presence of calcium and phosphorous were within the granular structures. Histological examination of capsular membrane did not reveal any evidence of calcium salts or special pathological exchanges. Further studies should be undertaken to determine the incidence and possible mechanisms of this phenomenon.