An increasing number of kindreds with thrombocytopenia have been described in the past decade as platelet counts have been done regularly. These patients are delineated from ITP, recognizing that some patients with hereditary thrombocytopenia may benefit from splenectomy. Until more information on platelet biochemical information is available, these disorders have been categorized by platelet size, now quantifiable as mean platelet volume (MPV). The availability of rapid and quantitative measurement of platelet count and MPV allow a better understanding of megakaryocyte proliferation and platelet production. The evolving changes of MPV during treatment and recovery from chemotherapy will now allow a forecast of marrow hypoplasia, since the platelet count and increase in MPV follow the increase in megakaryocyte ploidy to herald marrow regeneration. It is anticipated that the careful plotting of the MPV relationship to the platelet count will allow the physician to evaluate marrow function indirectly, in many cases replacing repeated bone marrow studies.