Medullary carcinoma of the thyroid (MCT) is a dominantly inheritable neoplasm derived from intrathyroid C cells. The cytogenetics of this tumor has been only sparsely and indirectly studied previously. This article describes the chromosomes of primary MCT tumor tissue cultured with colcemid for 48 hr, metastatic tumor in lymph node cultured for 7 days from the same patient, and of primary tumor tissue cultured for 3 weeks from a second patient. The modal numbers were 42-44 in all 3 specimens. Karyotypes from the metastastic tumor were similar to those of the primary tumor. Karyotypes of the two primary tumors, however, differed from each other, having in common only the modal numbers and the loss or structural alteration of a #22.