There has been much discussion about the effect of aging on routinely analysed haematological parameters. The consensus view favours a slow decline in the median value of haemoglobin, including the lower limit of the reference range, especially in the very old. This could be explained in part by the recent finding that many elderly patients with a normocytic, but not a microcytic or a macrocytic, anaemia have an impaired response to erythropoietin. In addition, the number of bony sites exhibiting active haematopoiesis decreases with age, with the marrow being replaced by fatty tissue. This involution first takes place in the long bones, particularly the femur, while the flat bones are relatively spared. The sternal marrow, for example, is one of the last bony sites to show fatty change, usually occurring after 60 years of age. Magnetic resonance imaging confirms this age-related reduction in marrow cellularity.