Studies dealing with the number or size of individual adipose cells in human bone marrow are lacking. To ascertain whether the age-related variations in fat tissue fraction depend on the size of individual adipocytes or their number or both, a stereological study of 20 normal human bone marrow specimens was performed. A total number of 17,039 adipose cell profiles was measured and two stereological parameters were obtained in each specimen: mean diameter and number of cells per mm3 of bone marrow. With increasing age, an increasing fat tissue fraction was observed (r= 0.61; p= 0.004). The fat tissue fraction correlated positively with the size (r= 0.81; p less than 0.001) and the number/volume (r= 0.59; p= 0.006) of adipocytes. The significance of both adipocyte size and adipocyte number/volume was confirmed by stepwise multiple regression, in which the size alone explained 66.2% of fat tissue fraction and both size and number/volume explained 97.2% of fat tissue fraction. These results are discussed from a pathophysiological point of view.