We performed an immunohistological study using biopsy samples of bone marrow obtained from patients, ranging in age from a newborn baby to 100 years old. Those patients suffering from hematological diseases or diseases that would be capable of affecting hematopoiesis were not included in the present study. The cellularity of the bone marrow did not change significantly with age during the first and the eighth decades, while the oldest group, ranging in age from 80 to 100, revealed significantly low cellularity. Proliferative activity assessed by Ki-67-positive cells was high in the middle-aged group and declined slightly in the elderly group. It was of interest to note that the percentage of apoptosis was relatively low in the young and middle-aged group, but significantly increased in the elderly group. The percentage of T cells did not change greatly between the first and the fifth decades, peaked at the sixth decade and gradually decreased thereafter. The percentage of B cells was about 10% at the first decade, decreased thereafter until the third decade, then increased again showing a peak at the sixth decade, and decreased thereafter. The percentage of CD68-positive cells was high in young patients, and decreased in the adult and elderly patients. The data in the present study suggest that hypocellularity in the bone marrow of elderly people could be ascribed partly to the increase of apoptosis, and might possibly be related to a decrease in the number of lymphocytes and macrophages, which would constitute part of the bone marrow microenvironment supporting hematopoiesis.