The bone marrow is commonly present in the medullary cavity of long bones in most mammals at birth. In the present study, the persistence of the osseous tissue was examined in the medullary cavities of piglet bones at and after birth. Immediately after birth, medullary cavities of long bones were almost completely filled with the spongy bone. The persisted spongy bone, i.e., medullary bone was prominent in younger pigs but decreased with advance in age. By 90 days of age, the medullary bone almost disappeared to form the extended medullary cavity. During the disappearing process of the medullary bone, osteoclasts showed significant increase in size and number (p less than 0.05). In irregular and flat bones, osseous trabeculae of the medullary bone became slender with age. The extramedullary hematopoiesis in the liver and spleen was very active till 30 days of age. This may suggest that the persistence of the medullary bone narrows the medullary cavity to cause poor medullary hematopoiesis and is compensated by extramedullary hematopoiesis. The relationship among the medullary bone, extramedullary hematopoiesis and piglet anemia was discussed.