The purpose of this study was to evaluate the radiotherapeutic management of 38 patients, with malignant lymphoma of the thyroid, seen at the Mayo Clinic between 1965 and 1979. There were 8 males and 30 females with ages ranging from 34 to 90 years (mean age of 65 years). A tissue diagnosis was made in all patients and tissue was available for reclassification under the "Working Formulation" in 31 of the 38 patients. Twenty-six patients had intermediate grade histology, four low grade and one indeterminate. Twenty patients were clinical Stage IE, 14 patients Stage IIE, one patient Stage IIIE, one patient Stage IV and two patients were unstaged. All patients were treated with approximately 4000 rad megavoltage irradiation (range 2400-6000 rad) to the neck only (10 patients) or neck and mediastinum (28 patients). Twenty patients received subdiaphragmatic radiotherapy and four patients received adjuvant chemotherapy. Median follow-up was 56 months with minimum follow-up of 30 months. Overall disease-free survival at five years was 59%. Of 14 patients who experienced a recurrence, 10 (71%) failed in two or more sites. The most common site of failure was in para-aortic lymph nodes. One year survival following recurrence was 29%; however, four of six patients receiving salvage therapy survived at least two years. Patients receiving radiation treatment to the neck and mediastinum and those with no gross residual disease at the initiation of radiotherapy were less likely to develop a recurrence. Patients receiving a planned break during the course of therapy did not have reduced overall disease-free survival. However, 4 of 20 patients (20%) who received split course therapy failed within the radiation fields compared to 2 of 18 patients (11%) who had no treatment break. Only 1 of 4 patients (25%) receiving adjuvant chemotherapy survived one year. Side effects of radiotherapy were minimal. We believe the radiotherapeutic management of clinical Stage IE and IIE primary thyroid lymphoma should include treatment of the neck, axillae and mediastinum to a dose of approximately 4000 rad using a continuous course technique. Additionally, gross total removal of the disease surgically may be beneficial.