Sixty-two patients with pterygium were treated by Sr90 postoperatively with doses of 1,000 reps weekly for 3 weeks and were evaluated by naked eye, slit-lamp, and cytologic examinations. Complications from beta irradiation were mild. Ten per cent of the patients had recurrences of the pterygium 2 to 18 months after completion of the first course of the treatment. Only 1.2 per cent of the pterygia recurred after the second course of therapy. Distinct cellular changes were observed in the irradiated conjunctiva. The changes were those of polymorphism, giant cell formation and chromatin abnormalities. Despite the persistent change of the cells, no serious sequelae in the conjunctiva were found as long as 11 years after radiation therapy.