Morphofunctional alterations to the platelets in myeloproliferative disorders (MPD), conditions featuring clonal rearrangement of the haemopoietic stem-cell, were examined. These platelet anomalies including morphological alterations, acquired storage pool disease, membrane alterations, altered arachidonic acid metabolism and structural alterations to the von Willebrand factor may cause thromboembolisms or haemorrhages that are responsible for a significant incidence of morbidity and mortality. In contrast the reduced mitogenic activity of the platelets may be of significant prognostic value since it proves the anomalous transformation of the megakaryocytic clone. The main in vivo and in vitro tests (bleeding time and the study of platelet aggregation) were investigate as indicators of platelet function. Unfortunately, these tests proved of little use for the early diagnosis of haemorrhage or thromboembolism in MPD.