Sebastian platelet syndrome is a rare autosomal dominant disorder characterized by macrothrombocytopenia with granulocyte inclusions similar to those in patients with Fechtner platelet syndrome but without evidence of hereditary nephritis and sensorineural hearing loss that characterizes the latter. Although by light microscopy the granulocyte inclusions in these disorders appear morphologically similar to those found in May-Hegglin anomaly, another autosomal dominant macrothrombocytopenia, by electron microscopy the inclusions are distinct. Studies of platelet function usually suggest normal or near-normal platelet function, although mild bleeding symptoms can be associated with each of these disorders. We describe a 38-year-old woman and her 11-year-old daughter who presented with lifelong histories of mild thrombocytopenia and easy bruising. Detailed hemostatic studies showed prolonged bleeding times in the child and the mother, with the child having absent secondary wave platelet aggregation responses to epinephrine, also reflected by testing with the platelet function analyzer (PFA-100 device). The mother's hemostatic studies were normal including platelet aggregometry, PFA-100 testing, and platelet flow cytometry. By light microscopy the blood smears of both individuals showed neutrophil inclusions, and their platelets were mildly enlarged but were not giant. Electron microscopy showed the neutrophil inclusions seen in classic Sebastian platelet syndrome or Fechtner platelet syndrome. These 2 cases expand the description of Sebastian platelet syndrome to include individuals with large but not giant platelets and mild or minimal thrombocytopenia. The differential diagnosis of hereditary thrombocytopenias is reviewed briefly.