This study investigated the relative contributions of pore size and pore density (number of pores per mm2) to porosity in the midshaft of the human femur. Cross-sections were obtained from 168 individuals from a modern Australian population (mostly Anglo-Celtic). The study group comprised 73 females and 95 males, aged from 20 to 97 years. Microradiographs were made of 100-μm sections and porosity, pore areas and pore densities determined using image processing software. The cortex was divided into three rings radially and into octants circumferentially, and the porosity, pore area and pore density of each segment were calculated. Results show that 81% of the variance in porosity can be explained by changes in mean pore area with only a further 12–16% explained by changes in pore density. These effects were found to be constant across all areas of the cortex and in both sexes. These results are significant in their consistency and ordered gradation and indicate a well-regulated and systematic process of bone removal with ageing. The results show a regular progression from less porous to more porous bone; this is a uniform process that occurs in all individuals, and factors such as sex and rate of ageing determine where on this continuum any individual is at a particular time.