Thirty-six unselected patients were investigated 3-24 years (mean, 7.8 years) after small-bowel resection for Crohn's disease (mean small intestinal resection, 105 cm). Iliac crest bone biopsies after in vivo tetracycline double-labelling showed a markedly reduced trabecular bone mass (controls, 0.25 +/- 0.06; patients, 0.15 +/- 0.05; p less than 0.01). The average bone remodeling and osteoid mineralization was normal, and only two patients demonstrated signs of frank but slight osteomalacia. The mean serum levels of the three vitamin D metabolites 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D, and 1,25-dihydroxyvitamin D were normal. The observed reduction in trabecular bone mass may theoretically be followed by an increased risk of spontaneous fractures.