A 44 year-old male patient was undergoing diagnosis and therapy for acute myelogenous leukemia (AML, M2). On physical examination a thyroid mass was palpated in the left lower lobe. He had palpitation and intolerance to heat. Thyroid function tests revealed hyperthyroidism T3: 150ng/dl (N:60-90), fT4: 2.26 ng/dl (N:0.70-1.80), TSH: 0.01 ulU/ml (N:0.25-5.00). Ultrasonography demonstrated a hypoechoic mass with scattered calcifications measuring 2.55 2.03 3.64 cm in size. F-18 FDG camera-based PET scan performed as a follow-up study of AML revealed a focal increased uptake in the left neck, where an autonomous nodule was detected on Tc-99m thyroid scan. After the diagnosis of toxic autonomous nodule, Goetz disease, he underwent surgical nodulectomy. Microscopically, the nodule contained follicular proliferation with degenerative change but without evidence of thyroid carcinoma. Focal uptake in autonomous thyroid nodules is due to increased glycolysis within the nodules. (Korean J Nucl Med 200135:286-287)