Iron deficiency in the elderly almost always results from blood loss. The loss of iron can be viewed as occurring in four stages, which are reflected in the different tests used to diagnose iron deficiency. Tests used to diagnose iron deficiency have certain limitations regarding their ability to detect iron deficiency before the overt anaemia occurs. The tests which diagnose iron deficiency most accurately are low serum ferritin and reduced iron staining of a bone marrow aspirate. Because iron is present in many metabolic processes besides the production of haemoglobin, iron deficiency results in a variety of defects which are manifested at biochemical, tissue, and functional levels. Iron is a component of several enzymes in the respiratory electron transport chain. Adequate haem and iron levels are necessary to control cytoplasmic and mitochondrial protein synthesis. Iron deficiency results in tissue defects, including those affecting the gastrointestinal tract, and defects of mitochondria and lymphocytes. Normal iron levels seem to be necessary for normal work capacity. A deficiency of iron, independent of the anaemia, results in reduced exercise capacity that can be measured in both physiological and economic terms. Elderly patients complaining of increased fatigue should therefore be screened for iron deficiency. There is evidence to suggest that iron deficiency may predispose individuals to certain infections. Other information points to the promotion of certain bacterial and parasitic infections after rapid correction of iron deficiency. Thus elderly patients having iron replacement therapy should be followed closely. A deficiency of iron has been shown to result in certain behavioural and learning abnormalities. Iron deficiency has been shown to result in impaired control of body temperature, resulting in an increase in catecholamine levels. The impairment in heat-generating ability was shown to result from reduced conversion of T4 to T3 in the peripheral tissues.