The incidence and significance of pulmonary megakaryocytes have been studied in the lungs of 100 autopsy cases of sudden death. Cases of sudden death are divided into two groups, that is, one group died of various accidents (58 cases) and another died of various diseases (42 cases)<BR>Results obtained are as follows :<BR>1) Pulmonary megakaryocytes are detected in all casses, but most of the cells have naked nuclei.<BR>2) The average number of pulmonary megakaryocytes is 29.2 in all cases of sudden death, 28.0 in the cases of males (71 cases), and 32.0 in cases of females (29 cases) .<BR>3) The average number of pulmonary megakaryocytes is 30.3 in the cases of accidentdeath, 31.8 in the cases of males, and 27.0 in the cases of females.<BR>4) The average number of pulmonary megakary ocytes is 27.7 in the cases of diseasedeath, 23.4 in the cases of males, and 41.5 in the cases of females. This marked difference between males and females may be explained by the fact that in the cases of females two cases with many pulmoary megakaryocytes (157 and 120, respectively) are included.<BR>5) With regard to the number of pulmonary megakaryocytes in the cases of accidentdeath, no marked differences are observed either between males and females or among various causes of death: the number ranges from 85.8 in the cases of subdural bleeding (4 cases, male) to 4.0 in the cases of both rupture of the liver (2 csses, male) and cresol poisoning (1 case, male)<BR>6) With regard to the number of pulmonary megakaryocytes in the cases of diseasedeath, no apparent differences are observed either between males and females or among various causes of death: the number ranges from 157.0 in the case of intestinal intussusception (1 case, female) to 3.0 in the case of subarachnoid hemorrhage (1 case, male) .<BR>Although marked difference in the number of pulmonary megakaryocytes is observed among individuals, regardless of accident or disease-death, causes of the difference are unknown.<BR>7) In the previous paper, we have reported that the numer of pulmonary megakaryocytes in the autopsy cases of hospital-death was 92.7 (79 cases) . In the present paper, we report that, in the cases of sudden death, the number of pulmonary megakaryocytes is 29.2 (100 cases) . Therefore, the number of pulmonary megakaryocytes in the cases of hospital-death is as 3.2 times as many than that in the cases of sudden death. Furthermore, in the cases of sudden plus disease-death, the number of pulmonary megakaryocytes is 27.7 (42 cases), which clearly shows marked difference between the cases of hospital-death and sudden death.<BR>These observations imply that, in the cases of hospital-death, the duration of disease, the various therapeutic treatment including surgical procedure and medication during hospita-lisation, and so forth, have some influence on the micro-environment of bone marrow, and facilitate the migration of bone marrow megakaryocytes out into the sinuses of bone marrow.