The goal of this study was to determine the cell origin of human pterygia. In order to determine the origin of these cells, longitudinal cryostat sections through five primary and two recurrent pterygia were studied immunohistochemically by finding limbal basal stem cell staining patterns as defined by monoclonal antibodies AE1 (staining positive) and AE5 (staining negative). In addition, sections were stained with antivimentin antibody. Altered limbal basal cells invading normal cornea along the basement membrane were identified in seven human pterygia with these specific monoclonal antibodies. A group of limbal basal cells (vimentin and AE1 positive) was always present between the dissolved edge of Bowman's layer and vascularized conjunctiva which contained goblet cells. Scattered patches of cells staining positive with both vimentin and AE5 (in addition to their AE1 staining) were also found in conjunctival epithelium growing on corneal basement membrane adjacent to the migrating limbal cells, indicating local infiltration by the altered limbal basal cells. This same pattern was also found in recurrent pterygia. Based on this data we propose that the pathogenesis of pterygia is due to a normal stationary parental limbal epithelial basal cell becoming altered and giving rise to a zone of motile daughter cells, the pterygium cells, which leave the limbal region and migrate as a group centripetally along the corneal basement membrane dissolving Bowman's layer. Since these altered limbal basal cells are found at the microscopic advancing edge over Bowman's layer with no fibroblast mass under them, the pterygium cell apparently precedes the rapid growth of the fibroblasts from the stroma.(ABSTRACT TRUNCATED AT 250 WORDS)