A pterygium is a non-cancerous lesion that usually grows slowly throughout life and starts in the clear, thin tissue (conjunctiva) of the eye. This growth covers the white part of the eye (sclera) and extends onto the cornea. It is often slightly raised and contains visible blood vessels. The problem may occur on one or both eyes. In rare cases, a pterygium can continue growing until it covers the pupil of the eye and interferes with vision. Pterygium is usually not a serious condition. But it can cause annoying symptoms such as a feeling of a foreign object in the eye.