242 members of 43 families with idiopathic haemochromatosis were investigated for increased body-iron stores in order to assess the value of serum-ferritin determination as a screening-test to detect preclinical disease. The serum-iron concentration was elevated in only 76% of relatives with increased iron stores, and it was also elevated in 10% of relatives with normal iron stores. The percentage saturation of transferrin was elevated in all relatives with increased iron stores but also in 33% of relatives with normal iron stores. Serum-ferritin was raised in 98% of relatives with increased iron stores and in only 3 (1.8%) of those with normal iron stores. These 3 subjects consumed alcohol in excess of 100 g ethanol per day, and their serum-ferritin levels fluctuated widely. Increased iron stores were reflected in increased serum-ferritin concentrations in subjects as young as 14 years in whom the liver-iron concentration was twice the normal upper limit and before there was any evidence of architectural damage to the liver. The serum-ferritin concentration is a useful non-invasive screening test for precirrhotic haemochromatosis.