In order to measure megakaryocyte DNA content in a greater number of well-defined patients, the use of bone marrow aspirates obtained postmortem is a basic requirement. We could show that the distinction between the ploidy classes in DNA histograms is possible until 18 h postmortem. Thus, bone marrow aspirates obtained up to 12 h after death can be expected to give reliable results. Megakaryocytes of the following patient groups were studied: 15 patients with metastatic tumors and paraneoplastic thrombosis, 15 patients with metastatic tumors without paraneoplastic thrombosis and 10 controls. An increased ploidization of the megakaryocytes was found in all 30 patients with metastatic tumors, independently of whether these patients suffer from thrombosis or not. Higher megakaryocyte ploidization, however, is correlated with a larger cytoplasmic mass of megakaryocytes, which leads to an increased platelet production. Besides an overcompensation for increased platelet consumption, a mitogenic or thrombopoietin-like factor produced by the tumor itself must be considered.