Gut permeability was studied in multiply injured patients with respect to the development of multiple organ failure (MOF). Two groups were defined according to MOF score (threshold 10 points) as to whether MOF developed (group 1; n = 11, four deaths) or did not (group 2; n = 21, no death). Gut permeability was determined from the ratio of urinary excretion of enterally administered lactulose and mannitol. Serum elastase concentrations were also determined. Mean(s.e.m.) gut permeability was abnormal during the entire study (day 1: group 1--5.1(2.1) versus group 2--10.6(4.1) (P not significant; P < 0.001 versus normal volunteers, 0.56(0.24)). An increase on days 3 and 5 correlated with serum elastase levels only in patients in group 1 (rs = 0.71, P < 0.01). Severe injury leads to increased intestinal permeability, which is related to a systemic inflammatory response.