The incidence and distribution of pulmonary megakaryocytes were studied in 100 unselected autopsy cases. Eighteen newborn infants were included in these cases. The causes of death were malignant neoplasms (44 cases), cardiovascular lesions (20 cases), inflammatory diseases (9 cases), and miscellaneous (27 cases) . The results were as follows ;
1. In 79 cases out of 82 adult ones (96.3%), pulmoary megakaryocytes were found in the pulmonary capillary bed. But the number of megakaryocytes was quite variable from case to case, that is, there was very great individual difference, the cause of which, however, was not clarifid.
2. In 8 cases out of 18 newborn infants, no pulmonary megakaryocytes were found and in 16 cases only 0 to 4 megakaryocytes were identified.
3. The distribution of megakaryocytes in each pulmonary lobe was almost equal, but the number of megakaryocytes in the right middle lobe was somewhat fewer.
4. Pulmonary megakaryocytes with distinct cytoplasm were very few, counting only ca. 1.7%, and the great majority had naked nuclei.
5. The difference of incidence of pulmoary megakaryocytes due to different diseases was not revealed.
6. The appearance of megakaryocytes in the pulmonary capillary bed may be considered as physiological event, as formerly insisted.
7. Whether pulmonary megakaryocytes do release platelets actually or not could not be clarified in. this study.
