Thrombotic thrombocytopenic purpura is a relatively new and distinct disease entity manifested clinically by purpura, thrombopenia, hemolytic anemia and bizarre transitory neurologic signs and symptoms. It is characterized histologically by widespread occlusion of the arterioles and capillaries with an amorphous or granular acidophilic material, endothelial proliferation within the vessels and the virtual absence of any inflammatory reaction in the walls of the blood vessel and adjacent tissues. Three cases occurring in children are reported together with a discussion of our present concepts of the disease. The clinical diagnosis of thrombotic thrombocytopenic purpura is not difficult provided the symptomatologic triad of thrombopenia, hemolytic anemia and neurologic disturbances are borne in mind. In some instances, diagnosis can be made prior to death by the finding of the characteristic histologic picture in paraffin sections of particles aspirated from the bone marrow or in biopsy specimens of the skin. It is stressed that the possibility of thrombotic thrombocytopenic purpura should be considered whenever an unusually severe or fulminating purpura, associated with thrombopenia, is encountered.