The function of the jejunum has been assessed in patients with ulcerative colitis (n = 23) and Crohn's disease of the colon (n = 20) by measurement of serum folate levels, oral folic acid and D-xylose absorption. Forty-six normal subjects served as controls. The mean serum folate level was 4.5 +/- 2.0 ng/ml in patients with the disease and 7.8 +/- 1.7 ng/ml in controls (p less than 0.001) and was similarly decreased in both ulcerative colitis and Crohn's disease patients. It was lower in patients under sulphasalazine therapy (n = 15) than in those untreated: 3.5 +/- 1.5 vs. 4.8 +/- 2.1 ng/ml (p less than 0.05). Serum folate correlated with disease activity in the latter only. The peak serum folate obtained during the oral absorption test was decreased in patients: 38.9 +/- 12.9 vs. 60.8 +/- 19.3 ng/ml in controls (p less than 0.001); this decrease was similar in ulcerative colitis and Crohn's disease, in treated and untreated patients and was independent of disease activity. Basal serum folate did not correlate with peak serum folate in any patient group. D-xylose absorption was normal in every case. Jejunal biopsies were performed in 23 patients, 13 of whom had folic acid malabsorption (13 with ulcerative colitis, 10 with Crohn's disease of the colon). The crypt height/villus height ratio was abnormal (greater than 0.6) in only 2 patients and borderline in 9 others. The fragility of enterocyte brush-borders and lysosomes, as assessed by biochemical methods, was normal in all cases.(ABSTRACT TRUNCATED AT 250 WORDS)