Subcutaneous fat necrosis in infancy is characterized by the appearance of subcutaneous nodules beneath a violaceous or erythematous plaque in infants exposed to ischemic stress from a variety of causes. It has been associated with hypercalcemia, hyperlipidemia and thrombocytopenia. This case of subcutaneous fat necrosis occurred 14 days after cardiac surgery in an infant taking enoxaparin who developed a paradoxical thrombocytosis. A potential etiological role for platelet sludging is discussed. Ultrasound images, not previously reported, showed thickened heterogeneous hypodense subcutaneous fat in the nodules and, thus, may be used to differentiate this condition from its surgical differential diagnoses.