The levels of anti-platelet factor 4 (PF4) antibodies, also known as anti-PF4 or heparin complex antibodies, are used to diagnose heparin-induced thrombocytopenia (HIT). In HIT, anti-PF4 antibodies induced by heparin exposure cause thrombocytopenia and thrombosis. However, anti-PF4 antibodies were recently reported to be associated with the development of fatal vaccine-induced immune thrombotic thrombocytopenia (VITT) after adenoviral vector vaccination for coronavirus disease 2019. HIT and VITT are caused by anti-PF4 antibodies and have similar pathological conditions. However, the severity of these conditions differs and the detection sensitivity of their antibodies varies depending on the assays used. Herein, we review HIT and VITT associated with anti-PF4 antibodies.