We made direct noninvasive magnetic measurements of hepatic iron stores with a specially designed superconducting quantum-interference-device (SQUID) susceptometer in 20 normal subjects and in 110 patients with liver disease, iron deficiency, hereditary hemochromatosis, or transfusional iron overload. Magnetic in vivo measurements of liver non-heme iron were closely correlated with chemical in vitro measurements in liver-biopsy specimens (r = 0.98, P less than 10(-5) up to 115 mumol per gram of liver tissue (wet weight) or more. Magnetically determined storage-iron concentrations were below 6.0 mumol per gram in iron-deficient patients and normal men and premenopausal women, but they were raised (9.7 to 31.4 mumol) in 12 of 67 patients with liver disease and were greatly increased (22.9 to 117.7 mumol) in patients with untreated hereditary hemochromatosis or transfusional iron overload. Magnetic measurements of iron stores provide a new quantitative technique for early detection of hereditary hemochromatosis and for rapid evaluation of treatment regimens for transfusional iron overload.