Bone marrow iron stores rise in proportion to the total body iron store in dietary iron overload. The situation in the genetic disorder of idiopathic hemochromatosis is not as clear. A method for measuring the storage iron concentration chemically on samples of bone marrow obtained by trephine needle biopsy was therefore developed. Its value as a measure of tissue iron stores was established in a preliminary investigation in which specimens of liver, spleen, and bone marrow were obtained at necropsies on 66 South African Negroes among whom dietary iron overload is common. A wide range of nonheme iron concentrations was found, but in each individual there was a highly significant correlation between the concentrations in the three tissues. Nonheme iron concentrations were then determined on trephine bone marrow biopsy specimens from eight Caucasian patients with untreated idiopathic hemochromatosis, and on percutaneous liver biopsy specimens from four of them. The concentrations in the livers were in the anticipated range of 5,000 mug per gram wet weight (2 per cent dry weight). In contrast the geometric mean value for bone marrow iron concentration was 186 mug per gram wet weight, a figure that fell below the fiftieth percentile for marrow iron concentrations in the South African Negroes, whereas the geometric mean liver iron concentration was above the ninetieth percentile. These findings indicate that subjects with idiopathic hemochromatosis whose liver iron stores are grossly increased do not show a comparable rise in bone marrow iron stores.