Clinical, haematological, biochemical, bacteriological, histological, and immuno-logical data of 100 patients with Crohn's disease have been examined with special reference to liver disease and dysfunction. Several significant points emerged from the clinical data. There was a high frequency of colonic involvement (57 per cent), and also a very high proportion of patients (48 per cent) with systemic complications, the commonest systemic complications being iritis (19 per cent) and sacro-ileitis (15 per cent). Nearly three-quarters (71 per cent) of the patients had been operated on at some time and 57 per cent had had one or more bowel resections. Local complications had occurred in 52 per cent of patients. Biochemical liver dysfunction was common, occurring in 26 (26 per cent) patients, the BSP being the test most frequently abnormal. Two patients with established chronic liver disease had considerable biochemical liver dysfunction, but about half the patients with lesser pathological changes in the liver had no biochemical liver dysfunction. Of the 39 patients who underwent liver biopsy, 19 (19 per cent) had pathological changes in the liver, the commonest changes found being pericholangitis (8 per cent), focal necrosis (6 per cent), and fatty change (4 per cent). Only two patients had chronic liver disease, one patient having chronic active hepatitis and the other portal cirrhosis associated with haemosiderosis. Culture of liver tissue for bacteria or L forms was uniformly negative. Immuno-logical studies proved unrewarding in relation to liver disease and dysfunction in patients with Crohn's disease. However, two significant differences did emerge in comparing the results of the immunological tests in ulcerative colitis with those in Crohn's disease. Positive immunofluorescence tests to human colon were found in 12·7 per cent of patients with ulcerative colitis, compared with only 2·3 per cent of patients with Crohn's disease. There was also a significant difference in IgM levels, with low values of IgM more frequent in Crohn's disease and high values more frequent in ulcerative colitis; further research will be necessary to determine the underlying reasons for this difference. The finding that the hepatic changes in Crohn's disease are remarkably similar to those in ulcerative colitis, both in frequency and in type, leads to several possible explanations. First, ulcerative colitis and Crohn's disease may be different manifestations of a single disease. Secondly, the liver disease and the other remote complications, which are also similar in Crohn's disease and ulcerative colitis, may be a consequence of chronic inflammatory disease of the intestine, irrespective of its exact nature. Thirdly, ulcerative colitis and Crohn's disease may both be generalized diseases in which the brunt falls upon the intestinal tract. The aetiology of the hepatic lesions remains obscure but some deductions can be drawn from the present data. From clinical considerations, the overt liver disease appears to behave like a viral hepatitis which has become chronic. There is no evidence to support the view that homologous serum hepatitis from blood transfusion is the significant factor. Drugs do not appear to be of any great relevance. Immunological relationships are essentially negative. Portal bacteraemia appears to be an unlikely cause of the overt chronic liver disease encountered in the present study.