Five iron-accumulating adenomatous hyperplastic nodules (AHN) in cirrhotic livers were examined histologically, by quantitation of tissue iron, and by magnetic resonance imaging (MRI) in vitro. Histologically, one AHN was benign, while the other four contained malignant hepatocellular foci. Iron staining and quantitation of tissue iron revealed that AHN were rich in iron, but the malignant hepatocellular foci and surrounding regenerative nodules were relatively poor in iron. The MRI showed that AHN were hypointense while malignant hepatocellular foci and surrounding regenerative nodules were isointense. Thus, malignant hepatocellular foci within AHN corresponded to iron-poor isointense foci. These findings suggest that malignant hepatocellular foci arising in iron-accumulating AHN are resistant to iron accumulation and that iron-accumulating AHN can be recognized with MRI in vitro as hypointense and malignant foci as isointense.