Eleven patients with idiopathic hemochromatosis were subjected to percutaneous liver biopsy, and seven were rebiopsied after repeated phlebotomies. The liver tissue was examined by light and electron microscopy. Ultrastructural morphometry was also performed and iron content was determined. Initially, all biopsies displayed large iron-laden lysosomes in the hepatocytes. Lysosomal volume density was increased and correlated well with liver iron content (r = 0.84, p less than 0.005). Neither mitochondria nor the endoplasmic reticulum showed any ultrastructural changes, except in the necrotic cells of biopsies with the highest liver iron content. In these cases, iron-laden lysosomes were also encountered in the Kupffer cells. Following treatment, liver iron content and lysosomal volume density were normalized. More specifically, iron content was 14.1 +/- 2.1 micrograms Fe/mg protein before and 1.3 +/- 0.3 micrograms Fe/mg protein after treatment (p less than 0.001). Lysosomal volume density was 6.1 +/- 0.8% before and 1.8 +/- 0.2% after treatment (p less than 0.001). Hence, in the precirrhotic stage of idiopathic hemochromatosis, the first evident ultrastructural changes are in the lysosomal compartment. These changes correlate well with the iron overload, also in advanced stages of the disease, and are reversed after iron removal.