To assess the potential role of matrix metalloproteinases (MMPs) in the pathogenesis of pterygia by comparing the immunolocalization patterns of MMPs in altered limbal basal stem cells, activated fibroblasts, and areas of elastotic degeneration adjacent to the pterygia.Methods: Nine primary and 1 recurrent pterygia along with normal superior limbal-conjunctival tissue and cornea were immunostained with mouse monoclonal antibodies specific for MMP-1, MMP-2, MMP-3, MMP-9, membrane type 1 (MT1)-MMP (MMP-14), and membrane type 2-MMP (MMP-15).Results: Normal conjunctival, limbal, and corneal cells lacked significant immunostaining except for cell surface MT1-MMP. In contrast, altered limbal basal epithelial cells of the 9 primary and 1 recurrent pterygia immunostained for all 6 MMPs. Activated and altered fibroblasts associated with the pterygia immunostained primarily for MMP-1. In contrast, stromal areas of elastotic degeneration (pingueculae) showed variable immunostaining of MMPs.