An increased prevalence of osteoporosis has been observed in lactase-deficient subjects. This association has been attributed to an avoidance of calcium-containing dairy products by lactase-deficient subjects and/or an adverse affect of lactose malabsorption on calcium absorption. Because the lactose in yogurt can be digested and absorbed by hypolactasic subjects, we tested the ability of lactase-deficient subjects and controls to absorb calcium from milk and yogurt. Subjects ingested 270 mg of Ca plus 45Ca in 250 g of milk or 147 g of commercial, unflavored yogurt, and blood radioactivity was assessed at intervals over 24 h. Based on the areas under the blood radioactivity curves, lactase-deficient subjects and controls absorbed calcium equally well from yogurt and milk. Lactase-deficient subjects absorbed 45Ca from both sources at least as well as did the controls. While we found no evidence to indicate that calcium in yogurt is better absorbed than calcium in milk, yogurt remains an excellent source of calcium because this fermented product is well tolerated by lactase-deficient subjects.