Prognoses and prognostic factors in 74 patients with primary lymphoma of the thyroid gland were analyzed. From 1968 to 1991, 74 patients with non-Hodgkin's lymphoma originating in the thyroid gland were treated at Noguchi Thyroid Clinic Hospital. Patients consisted of 53 females and 21 males, with a median age of 64.5 years. Survival curves were calculated by the Kaplan-Meier method and significance was tested by generalized Wilcoxon test. The overall 5-year and 10-year survival rates were 81.8% and 62.5%, respectively. The survival rate in stage II disease was significantly (P = 0.004) lower than that in stage I. By the LSG classification, there was no significant difference in survival curves between follicular lymphoma and diffuse lymphoma. Using the Working Formulation, there was no significant difference in survival curves between cases with low and intermediate grade histology. Histologic subtype of the tumor did not appear to be a significant determinant of prognosis. Patients with vocal cord paralysis had poorer survival rates than others (P = 0.001). Extrathyroidal spread of the tumor by direct soft tissue invasion had a distinctly adverse effect on prognosis.