Karyotypes from seven patients with thyroid lymphoma were studied before treatment. When the cytogenetic results were correlated with the clinical data, it became evident that there are two distinct groups of patients with thyroid lymphoma. In a group of patients with solely numerical abnormalities the disease is associated with a rather long duration from the onset of goiter to the time of operation for lymphoma, with abnormal thyroid function, and with positive tests for antithyroid autoantibodies. Two of four patients in this group showed trisomy 22; the remaining two showed a loss of a sex chromosome. The other group of patients with solely structural abnormalities is associated with a short duration from the onset of goiter to the time of operation, with normal thyroid function, and with the trend of negative tests for autoantibodies. Two of three patients in this group showed a 14q+ abnormality. These results indicate that there are two distinct type of thyroid lymphoma, chromosomal changes of which are implicated as pathogenetic factors in lymphomagenesis of different mechanisms.