In Crohn's disease there are changes in the ultrastructure of the intestinal epithelium, not only in the inflammatory tissue but also in non-ulcerated areas and in non-involved resection margins. Vitamin A is known to influence the metabolism and differentiation of epithelial tissue and has also been reported to normalize the number of stools after ileocaecal resection for Crohn's disease. We gave vitamin A (150 000 U daily) for 2 weeks to 8 patients with severe but not acute Crohn's disease. Various clinical parameters and the mucosal permeability to different-sized polyethyleneglycols were investigated before and after the administration of vitamin A. Disappointingly, no patient derived any benefit from the vitamin A ingestion as regards subjective manifestations, clinical signs, or mucosal permeability. However, some patients with Crohn's disease have low plasma concentrations of vitamin A and should be offered replacement therapy.