A defect in the barrier function of the intestinal mucosa has been proposed as important in both the pathogenesis and systemic manifestations of inflammatory bowel disease. After colonoscopy, polymers of polyethylene glycol (PEG) with molecular weights of 414-810 (mean 600), were instilled in the descending colon of patients with ulcerative colitis (n = 17) and in controls without intestinal inflammation (n = 8). The patients with active ulcerative colitis (n = 6) had a significantly increased uptake of PEGs in the molecular weight range 458-810, measured as urinary excretion over the first 6 hours after instillation. The median values for their excretion were 2.85-3.80% of PEGs instilled compared with 0.32-0.94% for patients in remission (n = 11) (p < 0.05-0.01) and 0.17-0.60% for the controls (p < 0.05-0.01). The differences in absorption of PEG 414 did not reach the present level of statistical significance. There was a positive correlation between PEG absorption and the endoscopic and histological grading of inflammatory activity in the sigmoid colon (p < 0.01-0.001). These findings support a correlation between the presence of active inflammation and PEG absorption. There was little evidence to support the presence of a primary defect in the colonic barrier in patients with ulcerative colitis.