Prognostic factors in 79 patients with malignant lymphomas involving the thyroid gland were analyzed. These patients suffered from progressively enlarging goiter with over 6 months duration in 60% of patients. The age at operation ranged from 16 to 80 years (median, 58 years). Male to female ratio was 1:1.8. Serum tests for antithyroid antibodies were positive in 83% of the patients, who also showed histologic evidence of chronic lymphocytic thyroiditis. Histologically, 52 cases (66%) were germinal center cell tumors with follicular or follicular and diffuse pattern in 9 cases. By the Working Formulation, 5-year survival rate of immunoblastic type (IBL) (13%) was much poorer than those of intermediate (79%) and low grade cases (92%) (P less than 0.001). The cases of IBL usually had a goiter of short duration, and frequently presented as advanced disease.