A 72-year-old Caucasian woman was admitted for an elective left total knee arthroplasty. Her surgery was uncomplicated and she was discharged to a rehabilitation facility. Twelve days later, she developed acute shortness of breath followed by a syncopal episode. She was hypoxic and cyanotic, requiring hospitalization and intubation, and was subsequently diagnosed with bilateral submassive pulmonary emboli and bilateral lower extremity deep vein thrombosis. She was started on unfractionated heparin infusion. Within 24 hours of exposure, she had an acute decrease in platelet count to 48,000. Heparin was discontinued and argatroban was initiated due to concern for heparin-induced thrombocytopenia (HIT). Both quantitative enzyme immunoassay and functional assay confirmed the diagnosis of HIT. The patient had no prior lifetime heparin exposure. Given the absence of preceding heparin therapy, this case is consistent with the diagnosis of spontaneous HIT.