The characteristics of hepatocyte intramitochondrial filaments (IMF's) were studied in 16 patients undergoing jejunal ileal bypass for morbid obesity. At the time of bypass, all of the liver biopsies showed varying degrees of steatosis and IMF's were present in 13 of the 16 operative specimens. The IMF's were most prominent in the periportal cells, whereas the intracellular lipid was concentrated in centrilobular cells. Midzonal hepatocytes were intermediate in both respects. The IMF'S were more abundant in biopsies showing moderate steatosis than in those with marked lipid accumulation in which the IMF's were limited to hepatocytes in juxtaposition to portal triads. The IMF's appeared to originate from mitochondrial cristae. When steatosis resolved 12 to 18 months after intestinal bypass the prevalence of IMF's diminished also. It is hypothesized that the presence of IMF's is ultrastructural evidence of an adaption to an altered metabolic environment which resolves when the inciting influence is removed.
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