It is not our purpose to appoint but affirmative conclusions about histopathologic material from necropsies and patients' biopsies with various affeccions. However, comparing the results obtained through the present work, we may admit the following: 1. The impression is that, in malignant neoplasies, the number of megakaryocytes in the bone marrow is high; it is regular in the lungs and depends on the medullary megakaryocytopoiesis. 2. It was not possible to identify no megakaryocytes in other organs porceeding from the some patients; so it seems that probably there is not an extramedullary megakaryocytopoiesis in adults. Megakaryocytes from the bone marrow are large cells, and under this condition, they are very often traped in to the capillaires of the lungs, where they are almost always destroyed. 3. As for the cytologic study, we must assinalate that, in malignant neoplasies, megakaryocytes of the bone marrow are large cells, well conserved (strutured), morphologically normal, bi or polylobated, with polymorphous nuclei at different stages of maturation and presenting developed cytoplasm; in megakaryocytes found in the lungs, the nuclei are huge, extremely polymorphous, dark and piknotic, showing the most bizarre shapes, surrounded by a thin cytoplasmic layer; in many cases, the cytoplasm is not even apparent. Parallel results from a larger number of samples (reports) would be necessary before allowing any conclusion concerning the average of megakaryocytes in patients from malignant diseases.