PURPOSE: To study whether plica semilunaris (PS) temporal ectopia is associated with primary nasal pterygia and whether such hypothetical ectopia is linked to with-the-rule astigmatism so that a discussion of the clinical and surgical implications of such possible findings might take place. METHODS: Cross-section observational study of 208 consecutive eyes of 152 cases and 169 eyes of 109 control subjects enrolled for 6 months. Four PS position grades were designed. Grade 1 features a grossly vertical PS position. Grades 2 and 3 involve temporal displacement and an abnormal PS tilt. Grade 4 involves PS obliteration with possible associated temporal caruncle dragging. Pterygium corneal encroachment, pterygium fleshiness, and manifest refraction were recorded. RESULTS: PS position was significantly displaced temporally in pterygium-affected eyes (P = 2 x 10(-4)). Corneal encroachment (P = 2.1 x 10(-5)), pterygium fleshiness (P = 2.7 x 10(-5)), and age (P = 1.3 x 10(-2)) were positively correlated with PS position grades > 1. The presence of with-the-rule astigmatism was significantly linked to PS position grades > 1 (P = 0.01). CONCLUSIONS: Primary nasal pterygia result in PS temporal ectopia, and PS position grades (more than grade 1) are linked to the presence of with-the-rule astigmatism. These findings are consistent with the exertion of tractional forces at both pterygium ends. In eye abduction, medial canthus structures (PS, caruncule) may be dragged temporally. Thus, pterygium-related corneal traction may not always significantly increase in eye abduction. PS temporal ectopia may influence the decision to remove PS during pterygium surgical excision.