We have studied the liver histologies of 106 severely obese patients and the effects of jejunoileal bypass 2–5 years later in 12 of them when a second operation was needed for conditions not influencing liver function. Liver specimens of severely obese patients showed fatty infiltration (95%), inflammatory infiltration (72%), and hepatic fibrosis (50%). Ultramicroscopic analysis of liver biopsy samples from patients who had jejunoileal bypasses showed significant worsening of the fatty infiltration and hepatic fibrosis, whereas inflammatory infiltration was not changed. There were many new collagen fibrils. The smooth endoplasmic reticulum was hypertrophic and dilated. The mitochondria showed some signs of matrix degeneration. There were many lipofucsin granules and vacuoles containing dense osmophilic bodies. Our data suggest that liver histology should be evaluated in patients selected for jejunoileal bypas