We report three case reports of different, less frequent causes of thrombocytopenia--autoimmune thrombocytopenic purpura (AITP), thrombotic thrombocytopenic purpura (TTP) and heparin-induced thrombocytopenia (HIT). Pathophysiology of these conditions is also discussed. AITP is an autoimmune disorder characterized by destruction of thrombocytes by autoantibodies targeted to platelet antigens. The cause of thrombocytopenia in TTP is an endothelial disorder with subsequent formation of platelet aggregates. HIT type I is a frequent, mild form of thrombocytopenia caused by reversible aggregation of thrombocytes following to heparin administration. HIT type II is a late, more severe form of thrombocytopenia caused by immunologic mechanisms.