[en] Seventy-four (4.4%) of the 1680 Segment III beagles had one or more thyroid tumors as of December 31, 1979. Four sacrificed at 5 years of age and 13 sacrificed at 8 years of age had thyroid tumors. Ten malignant thyroid tumors have been removed surgically and 47 other dead dogs with thyroid neoplasms were found. Most tumors were of thyroid follicular cell origin and were near equally divided between benign and malignant varieties. Five dogs had tumors of the parafollicular cells (C-cells). To date, thyroid neoplasia does not appear to be associated with radiation exposure, but there does not appear to be an association with hypothyroidism. Forty-seven (68%) of the 69 dogs with thyroid follicular cell tumors were hypothyroid