A premature infant boy of 35 weeks and 4 days weighing 2605 g was found to have bilateral supraclavicular masses at 13 days of life. He had persistent pulmonary hypertension of the newborn and thrombocytopenia. Delivery was by cesarean section for nonreassuring fetal status, and Apgar scores were 5 at 1 minute and 8 at 5 minutes. Initial radiographs of the chest demonstrated abnormal supraclavicular fullness bilaterally (Figure ​(Figure11), and soft tissue neck sonography demonstrated hyperechoic masses involving the supraclavicular region bilaterally with internal vascular flow and hyperechoic bands. Differential considerations included lipoma, lipofibroma, hemangioma, and lymphangioma. Figure 1 Chest radiograph demonstrates supraclavicular and axillary “fullness” and fatty density (arrows). Computed tomography (CT) of the chest and neck were subsequently performed to evaluate the masses for the presence of fat (Figure ​(Figure22). Hypodense masses were demonstrated within the axilla, supraclavicular fossa, and lower neck bilaterally. Peripheral nodular enhancement was present within the axillae. Differential considerations included mixed venolymphatic malformation, hemangioma, and lympho-proliferative disorder. There was no evidence of lymphadenopathy within the chest or neck. Figure 2 CT of the chest demonstrates bilateral fat density (arrows) with nodular enhancement (arrowheads) in the axilla and along the lateral chest. Subsequent magnetic resonance imaging (MRI) of the chest demonstrated symmetric and confluent soft tissue extending from the neck inferiorly to the chest (Figure ​(Figure33). This soft tissue demonstrated intermediate signal intensity and was homogeneously isointense to muscle on T1-weighted images. This tissue was homogeneously hyperintense to muscle and hypointense to fat on T2-weighted images. Figure 3 MRI images of the chest and neck. (a) Coronal T2-weighted image demonstrates a bilateral symmetric supraclavicular and axillary confluent mass (arrows). (b) Axial T1-weighted image demonstrates bilateral symmetric intermediate signal intensity mass at ... What is the most likely diagnosis? DIAGNOSIS: Brown adipose tissue (BAT).