Animal studies have indicated that calcium absorption is increased by lactulose, a synthetic disaccharide. Therefore, the influence of lactulose on calcium absorption was measured in postmenopausal women who may benefit from the possible enhancing effect of lactulose on calcium absorption. Twelve postmenopausal women drank 100 ml of water containing 5 or 10 g of lactulose or a reference substance at breakfast for 9 days. The three treatments were given according to a randomized, double-blind, cross-over design, separated by two 19-day wash-out periods. On the 8th day of each treatment period, 44Ca dissolved in orange juice was drunk immediately after the solution with the study substance and just before a standard breakfast with 162 mg of carrier calcium. Within half an hour, 48Ca was given intravenously. Based on isotope ratios measured in urine collected before and until 36 h after isotope administration, true fractional calcium absorption was calculated. Calcium absorption during the treatments with the reference substance, 5 g and 10 g of lactulose was (mean ± SD) 27.7 ± 7.7, 30.0 ± 7.6, and 32.2 ± 7.0, respectively. A significant difference in calcium absorption was found between the highest dose of lactulose and the reference treatment (p < 0.01). A significant linear trend was found between the dose of lactulose and its positive effect on calcium absorption. In conclusion, in postmenopausal women a 9-day consumption of lactulose increases calcium absorption in a dose- response way. More research is warranted to explore how lactulose stimulates calcium absorption and whether it is able to improve calcium balance and/or to attenuate the rate of aging bone loss.