This paper reviews the histological and epidemiological characteristics of pterygium which suggest that chronic exposure to ultraviolet radiation plays an important role in the pathogenesis of this disorder. However, there is conflicting evidence indicating that chronic inflammation from other causes can induce the disease. The results of two epidemiological surveys undertaken in Southern Africa show that pterygium is not closely linked to other chronic actinic disorders such as pinguecula and climatic droplet keratopathy. Pterygia differ from these disorders by their vascularity which is probably induced by chronic inflammation. On histological examination, we found that excised pterygia contained a lymphocytic infiltration consisting predominantly of T cells. In the pathogenesis of pterygium we believe that chronic irritation (from whatever cause) produces a chronic inflammatory cell infiltration with resultant inflammatory oedema, attempt at repair and cell induced angiogenesis. These processes, together with actinic damage, are responsible for the fibrovascular reaction so characteristic of a growing pterygium.