A 27-yr-old woman with active Graves' disease, who underwent total thymectomy, is reported. Removal of the hyperplastic thymus (140 g) did not alter the course of her hyperthyroidism. Serum LATS activity remained elevated. Peripheral lymphocytes and thymocytes were cultured with phytohemagglutinin (PHA); LATS activity was only detected in the culture medium of the peripheral lymphocytes. Both cell types showed good viability as indicated by their 3H-thymidine uptake. LATS and TSH were undetectable in saline extracts of the thymus. Evidence is presented that this hyperplastic thymus was not involved in the pathogenesis of Graves' disease in this patient.