Between January 1982 and March 1987, 23 patients (26 orbits) were treated for orbital pseudotumor with radiation therapy at the Department of Radiation Oncology, Hospital of the University of Pennsylvania. The patients were referred for clinical relapse after steroid taper in 70%, no response to steroids in 17%, and no steroid treatment (refused or contraindicated) in 13%. Presenting symptoms/signs included soft tissue swelling in 92% of orbits, pain in 92%, proptosis in 85%, and extraocular muscle dysfunction or ptosis in 69%. Decreased visual acuity was seen in only 19% of orbits. Biopsy was performed in nine patients. Treatment consisted of 2000 cGy in 2 weeks in 10 fractions for all patients. Median follow-up was 41 months, with a mean of 53 months, and a range of 21-92 months. Complete response was documented in 87% of orbits with soft tissue swelling, 82% with proptosis, 78% with extraocular muscle dysfunction, and 75% with pain. Of the five patients with visual acuity defects, three experienced complete recovery. There was no difference in complete response in patients biopsied versus those not biopsied. Overall, 17 orbits have remained in complete orbital response with no further steroid requirement (66%). Three orbits suffered local relapse at some point following radiation therapy and were retreated with steroids. These three orbits had durable local control off steroids at last follow-up (11%). Therefore, 77% of orbits attained durable local control and were steroid independent with radiation therapy alone or radiation therapy followed by steroids for relapse. Only one patient developed systemic lymphoma with follow-up. No pretreatment clinical factor reached statistical significance with respect to prognosis following radiation therapy at the less than or equal to .05 level. There were no significant acute or chronic side effects secondary to treatment. Steroids should continue to be first line treatment for orbital pseudotumor, but radiation therapy has a well-defined role in cases of steroid failure or in patients unable to tolerate steroid therapy.