A health examination survey showed that 5.7% of 2322 middle-aged men had a haematocrit value below 40%. Only 1.2% had two consecutive low values. 14 men had no bone marrow haemosiderin and increased their haematocrits following iron therapy. In 13 men with no cause of the anaemia detected there was also a significant increase in haematocrit following therapy. The true prevalence of iron deficiency anaemia in this population may be estimated to about 1%.