BACKGROUND Posterior capsule opacification (PCO) is the most common complication of lens extraction. Although intraocular lenses (IOLs) are thought to inhibit capsule opacification, the mechanisms by which they do this are poorly understood. This study was done to determine the effects of pseudophakia on secondary cataract and PCO in experimentally lentectomized dogs. METHODS Twenty-four normal dogs were bilaterally lentectomized by phacoemulsification and unilaterally implanted with a plano-convex polymethylmethacrylate IOL. Secondary cataracts and capsule opacification were evaluated at weeks 1, 2, 4, 10, 14, and 20 after surgery by retrolillumination photography, light microscopy, and scanning and transmission electron microscopy. RESULTS The pattern of secondary cataract and PCO in dogs was found to be similar to that in other animal species. Production of new lens material was prominent in the equatorial region, and PCO resulted from fibrous metaplasia of lens epithelium and subsequent capsular fibrosis and wrinkling. The presence of an IOL did not prevent the posterior migration of epithelium, nor did it prevent fibrous metaplasia. The presence of an IOL did, however, minimize the capsule-wrinkling effects of fibroplasia and limit the space available for lentoid formation. CONCLUSIONS In pseudophakic eyes, IOLs influence secondary cataract formation by limiting the space available for lentoid formation and by maintaining a linear scaffolding for lens epithelial fibrous metaplasia.