A 19 year-old male was referred to our department because of macrothrombocytopenia. His platelet count was 73,000/microliter and giant platelets were observed in the peripheral blood smear specimen. Though he had been suffering from severe atopic dermatitis for four years, he seemed to be healthy without bleeding tendency. When he underwent a shunt operation for tetralogy of Fallot without any complication at nine-years old, thrombocytopenia was allegedly pointed out for the first time. Bone marrow aspiration revealed no abnormal findings with no chromosomal aberration. Normal platelet aggregation responses against adenosine diphosphate, epinephrine, collagen, and ristocetin were observed. The platelet adhesiveness (modified Salzman method) was slightly elevated. Unlike other reported syndromes associated with macrothrombocytopenia, his leukocytes had no inclusion bodies. His mother also had macrothrombocytopenia thus, this disorder was suspected to be hereditary.