The aim of this study was to determine serum retinol levels in patients with inflammatory bowel disease and to attempt to elucidate the mechanism of changes in vitamin A metabolism in these disorders. It was found that in 15 patients with active ulcerative colitis, 14 patients with active Crohn's disease and in 3 operated patients with recurrent Crohn's disease serum retinol levels and retinol-binding protein were significantly lower than in controls. Concentrations of vitamin A did not depend on the localization of inflammatory bowel disease, previous ileal resections, duration of the disease or age and sex of the patients. During successful treatment of active ulcerative colitis normalization of serum retinol levels without substitution of vitamin A was observed. Repeated determinations in patients with Crohn's disease who had low serum retinol levels in an active phase of disease revealed normal vitamin A levels in an inactive phase. The absorption of vitamins A and E in patients with inflammatory bowel disease was normal. The normal serum retinol concentrations in patients with diarrhea due to irritable bowel syndrome, and in those with anorexia nervosa exclude the influence of diarrhea and body weight itself on vitamin A levels. The results of this study indicate that serum retinol levels in patients with active inflammatory bowel disease are secondary to the decreased serum retinol-binding protein concentrations, and probably depend on the increased protein catabolism in these disorders.