Calcium absorption and endogenous loss of calcium were measured in a group of patients with Crohn's disease, using a simultaneous metabolic balance and calcium isotope regimen. Calcium malabsorption resulting in negative calcium balance was found in only 4 of 31 patients with Crohn's disease. No elevation of endogenous fecal calcium or total secreted intestinal calcium was observed in 10 patients studied, regardless of the level of net or true calcium absorption. Correlation between calcium balance and serum protein loss was observed, but no association was noted with intestinal fat excretion, d-xylose absorption, bacterial colonization of the jejunum, or glucocorticosteroid therapy. The results indicate that in this group of patients with Crohn's disease involving different areas of the intestine, calcium malabsorption occurred infrequently and that the levels of calcium excretion correlated best with enteric protein loss.