Calcium-binding protein (CaBP) (molecular weight, 10,000) was measured in small-intestinal biopsy specimens from 36 patients with malabsorption syndromes: short-bowel syndrome (n = 13), untreated coeliac disease (n = 4), coeliac disease in remission (n = 7), patients with intestinal bypass owing to morbid obesity (n = 5), and in patients with chronic diarrhoea of unknown cause (n = 7). Twelve patients with no signs of malabsorption who had the irritable bowel syndrome were used as controls. Patients with small-bowel resections showed reduced concentrations of CaBP (p less than 0.01) and low intestinal calcium absorption (p less than 0.05). Small amounts of CaBP were found in intestinal specimens from patients with coeliac disease in remission (p less than 0.01), and CaBP was almost undetectable in patients with a newly diagnosed coeliac disease and avillous jejunal biopsy findings (p less than 0.001). Patients with chronic diarrhoea and patients with an intestinal bypass had CaBP concentrations comparable to those of the control group. A direct correlation was found between CaBP and the fractional calcium absorption in all patients (p less than 0.05). CaBP may therefore be considered an indicator of the efficiency of the small intestine to absorb calcium.