Antiphospholipid syndrome (APS) is an autoimmune disease caused by antiphospholipid antibodies. At our institution, APS is diagnosed on the basis of the Sapporo criteria, which consist of thrombosis and recurrent pregnancy-related complications and the following laboratory findings: the presence of lupus anticoagulant, anticardiolipin antibody, or anti-β2 glycoprotein 1 antibody. However, we sometimes treat patients we strongly suspect of having APS but who do not satisfy the laboratory criteria. To accommodate such suspected cases, a subtype of APS termed seronegative APS has been proposed. Here, we report on a man with chronic thromobocytopenic purpura since the age of 3 years and multiple cerebral infarctions since the age of 14 years who finally received a diagnosis of seronegative APS with positive antiphosphatidylethanolamine antibodies.