Plasma levels of 25-hydroxyvitamin D (25-OHD) were found to be significantly reduced in a group of patients with small bowel resection when compared with normal controls. Plasma levels of 25-OHD after an oral dose of 25-hydroxyvitamin D3 (25-ohd3) were also reduced in the patient group. Dietary intake of vitamin D tended to be low in many patients, but seasonal variation of plasma 25-OHD levels indicated normal exposure to sunlight in most of the patients studied. It is suggested that these studies provide some evidence for malabsorption of 25-OHD after small bowel resection, which, it is postulated, may be due to interruption of the enterohepatic circulation of bile acids and of 25-OHD. These factors may contribute towards the low plasma levels of 25-OHD found in the patient group.