Increased intestinal permeability has been proposed as one aetiological factor for inflammatory bowel diseases (IBD). We have previously found that intestinal permeability of a water-soluble contrast medium, iohexol, correlates with disease activity. The objective was to compare the iohexol test with the lactulose-mannitol ratio, which is a more extensively studied permeability marker, in patients with active IBD. Urinary excretion of iohexol was compared to the lactulose-mannitol ratio in 22 patients with an exacerbation of IBD and in 10 healthy controls. Median intestinal absorption of iohexol was 0.64% (range 0.13-3.8%) in the 22 patients and 0.3% (range 0.15-0.54%) in the controls (p = 0.016), whereas the median lactulose-mannitol ratio was 0.037 (range 0.01-0.260) in patients and 0.03 (range 0.004-0.063) in controls (N.S.). Correlation between urinary excretion of iohexol and lactulose-mannitol ratio was positive (R = +0.41, p = 0.018). The urinary excretion of iohexol correlated positively with endoscopic disease activity (R = +0.74, p < 0.001) and the modified Harvey-Bradshaw index (R = +0.44, p = 0.04). The lactulose-mannitol ratio correlated positively with endoscopic disease activity (R = +0.44, p = 0.05), but correlations with clinical index or c-reactive protein were poor. In conclusion, the iohexol test is a superior activity marker compared to the lactulose-mannitol ratio which probably reflects, instead, some pathogenic property of IBD.