Samples of gall bladder bile from patients with and without carcinoma of the gallbladder were examined for lipid peroxidation (free radical oxidation) products. 4-hydroxynonenal (HNE) was found in higher concentration in the bile of patients with carcinoma of the gall bladder. There was also a highly significant difference in the concentration of HNE among carcinoma of the gall bladder, cholelithiasis and control group of patients with miscellaneous abdominal disorders. This is the first study of lipid peroxidation in carcinoma of the gall bladder.