We have reviewed 37 patients with primary orbital lymphoma, using the Ann Arbor criteria and the Working Formulation and its modification. Thirty-one patients had stage I disease, four stage II, one stage III and one stage IV. The male to female ratio was 2.7:1. There were 34 low-grade tumours (including 24 mantle zone) and three intermediate-grade. Patients were divided into three groups according to their primary treatment. Group 1: radiotherapy (17 cases); Group 2: surgery alone (13 cases); Group 3: chemotherapy (seven cases). Patients were followed up from 5 months to 24.3 years, with a mean and median of 7.6 and 6.2 years, respectively. The BMDP software package was used for survival estimation (Kaplan-Meier) and determination of prognostic variable (univariate Cox regression). Local relapse-free survival at 10 years was 100% in Group 1, 0% in Group 2 and 42% in Group 3 with a statistically significant difference (p < 0.01) in favour of radiotherapy. Statistically significant good prognostic features were: complete remission (CR) in response to initial treatment, primary radiotherapy and older age. For stage I cases, there was no difference in distant relapse-free survival in the three groups. The overall cause-specific survival for stage I patients at 10 years was 100% for each group and at 20 years was 100, 67 and 0% for Groups 1, 2 and 3. The difference between the primary radiotherapy and chemotherapy-treated groups was significant at the p = 0.08 level. Statistically significant prognostic factors were early stage, low-grade histology and primary radiotherapy. In one patient, ptosis and diplopia appeared after surgery. One case of glaucoma required enucleation, one patient suffered severe dry eye syndrome. All patients (11/11) in whom the lens received direct radiation developed cataracts of different degrees if follow-up was long enough. Cataract formation was prevented by adequate lens shielding. One patient in CR from a stage I low-grade tumour died from chemotherapy-induced marrow aplasia. Primary orbital lymphoma is an indolent, usually stage I disease, showing low to intermediate-grade histology. After biopsy the best treatment is 30 (low-grade) to 40 Gy (intermediate-grade) carefully planned, lens-sparing megavoltage radiation without adjuvant chemotherapy.