PURPOSE To assess the outcome and prognostic factors in patients with orbital lymphoma treated by radiotherapy (RT). METHODS AND MATERIALS Between 1980 and 1999, 90 consecutive patients with primary orbital lymphoma were treated in 13 member institutions of the Rare Cancer Network. A full staging workup was completed in 56 patients. Seventy-eight patients had low-, 6 intermediate-, and 6 high-grade lymphoma, and 75 had a single orbital localization. All patients underwent RT with a median dose of 34.2 Gy (range 4.0-50.4). Eleven patients received chemotherapy in addition to RT. RESULTS After RT, local control was achieved in 97% of the patients. Local progression occurred in 2% and local relapse 1%. The rate of systemic relapse was 20%, and 9% of the patients developed metachronous contralateral eye involvement. The 5-year disease-free survival, overall survival, and cause-specific survival rate was 65%, 78%, and 87%, respectively. In univariate analyses, the statistically significant favorable prognostic factors were younger age, low grade, normal erythrocyte sedimentation rate, absence of muscular infiltration, complete response to treatment, conjunctival localization, and normal lactate dehydrogenase value for overall survival, disease-free survival, and freedom from treatment failure. In multivariate analysis, the favorable factors were younger age and low grade for overall and disease-free survival; a favorable response, conjunctival localization, and complete staging were highly significant for disease-free survival and freedom from treatment failure. Neither the RT technique nor the total dose influenced the outcome. Cataract and xerophthalmia were the most prominent late toxicities. CONCLUSION Moderate- to low-dose RT alone is able to control primary orbital lymphoma with low morbidity. A full staging workup is warranted in these patients. Prognostic factors were identified that could be useful in the overall management of this uncommon site of primary lymphoma.