The formation of a pterygium is modelled using a population balance model of corneal and limbal epithelial production. Scattered light incident at the temporal limbus is focused at the nasal limbus with a peak intensity > or = 20 times. We hypothesize that this causes a focal alteration of corneal epithelial stem cells (which play a role in maintaining a barrier between corneal and conjunctival epithelia). Normative rabbit corneal data were used. Light-induced conjunctival epithelial proliferation was implemented by amplifying normal epithelial cell production along a nasal sector of the limbal circumference. A ten-fold normal peak in stimulation was assumed at the nasal zero azimuth (3 o'clock position in the right eye) with a quadratic attenuation to zero stimulation (normal) at the circumferential limits. Adjacent points on the outer limbus were assumed to be independent generators of epithelial cells that migrated in centripetal streams into the cornea. Normalization of the attrition rate for a net surface accumulation of the proliferating cells was embedded into the computational algorithm. The localized conjunctival mass was allowed to travel the predicted distance along the surface before the final shape was computed. The result shows a wing-shaped mass with a curved leading edge (corneal side). It is proposed that the initial biologic event in pterygium pathogenesis is an alteration of limbal stem cells due to chronic ultraviolet light exposure. The concomitant breakdown of the limbal barrier and subsequent conjunctivalization of the cornea explain the shape and formation of a primary pterygium.