A series of 10 patients with optic nerve glioma involving the chiasm is presented, all treated by radiotherapy. In a follow-up period varying from six months to 17 years, none has died of the tumour and four have shown improvement of vision. In an attempt to determine whether or not treatment with radiotherapy is beneficial the results of six other series, where the necessary information is availabe, have been pooled. This tumour often behaves in a benign fashion, especially in patients under 20. Neither surgery nor radiotherapy show any obvious effect on survival. Any attempt at surgical removal, even just biopsy, may well make vision worse. Spontaneous improvement of vision can occur, but improvement appears to be more likely after radiotherapy, which is probably the management of choice in most cases.