Radiotherapy offers an alternative to steroid therapy for idiopathic inflammatory orbital pseudotumours. Twenty-four orbits were treated with radiotherapy, of which 75% responded adequately. There were no significant complications and no recurrences during the average follow-up time of 22.3 months (1-50 months). Biopsy, CT scanning, and steroid response of orbital pseudotumours can be helpful in predicting the response to radiotherapy. An adequate response to radiotherapy can be predicted if lymphocytes with germinal centres are present in biopsy specimens. Polymorphonuclear leucocytes and eosinophils suggest a poor response to radiotherapy. Small, discrete, inflammatory masses respond better than large diffuse masses. Pseudotumours initially responding to steroids were likely to respond to radiotherapy.