OBJECTIVE To compare the formation and rates of posterior capsule opacification (PCO) in eyes with poly(methyl methacrylate) (PMMA) versus silicone intraocular lenses (IOLs) using an in vivo objective method. METHODS Taipei Municipal Yang-Ming Hospital, Taipei, Taiwan, Republic of China. METHODS This prospective study comprised 40 eyes with senile cataract receiving phacoemulsification with capsular implantation of an IOL from March to April 1997. The uneventful surgeries were performed using the stop and chop method by the same surgeon. Twenty eyes received a PMMA IOL (Pharmacia 812A) and 20, a silicone IOL (AMO SI-30NB). One year later, digital retroillumination images taken with the EAS-1000 anterior segment analysis system (Nidek) were used to analyze posterior capsule transparency over the central 3.0 and 5.0 mm optic zones and evaluate the degree of PCO over the central and peripheral zones. RESULTS Over the central 5.0 mm optic zone, mean transparency of the capsule was 87.71% +/- 11.35% (SD) in the silicone group and 79.22% +/- 21.10% in the PMMA group (P = .17). Over the central 3.0 mm optic zone, the means were 97.17% +/- 5.96% and 86.32% +/- 19.60%, respectively (P = .048). Mean opacity in the central zone was 75.18 +/- 13.22 digital opacity units (OUs) in the silicone group and 80.24 +/- 7.93 OUs in the PMMA group (P = .18). The means in the peripheral zone were 88.49 +/- 18.47 OUs and 90.35 +/- 9.98 OUs, respectively (P = .71). CONCLUSIONS The posterior capsule in the silicone IOL group was more transparent than in the PMMA IOL group over the central 3.0 mm optic zone after 1 year follow-up.