Liver morphology and biochemistry were investigated in 61 morbidly obese subjects selected by defined criteria. Median overweight was 82 per cent (range 61 to 170 per cent), and median duration of overweight was 20 years (range two to 45 years). No patient had more than a moderate alcohol consumption and only one was diabetic. Four biopsies (7 per cent) showed normal liver tissue, while fatty change was the main diagnosis in most cases (85 per cent). Increasing degrees of fatty change was significantly (P less than 0.02) associated with presence of lipogranulomas (found in 54 per cent of the biopsies), focal necroses (found in 28 per cent), slight parenchymal inflammation (found in 33 per cent), and Kupffer cell proliferation (found in 49 per cent). Slight portal inflammation was seen in 23 per cent but portal fibrosis in only 2 per cent of the biopsies. No case of cirrhosis was registered. Patients with moderate or severe fatty change, lipogranulomas , focal necroses or with parenchymal inflammation were significantly more obese than patients without these changes (P less than 0.05). Even in absence of fatty change, obese subjects showed a markedly decreased serum albumin concentration and an elevated serum alkaline phosphatase activity (P less than 0.0001) compared with non-obese controls. Serum lactate dehydrogenase and aspartate aminotransferase were significantly raised only in patients with fatty change. With respect to serum bilirubin and plasma cholesterol concentrations no significant differences were detected between patient subgroups and controls.(ABSTRACT TRUNCATED AT 250 WORDS)