To determine whether lactose influences the absorption of calcium, the uptake of calcium from lactose-hydrolyzed milk and from unhydrolyzed milk was measured in 20 adults: 10 were lactase-deficient and 10 were lactase-sufficient as defined by breath hydrogen test, plasma glucose determination after oral lactose dose, and presence or absence of symptoms after lactose ingestion. On different days, each subject received either lactose-hydrolyzed or unhydrolyzed milk. Calcium absorption was measured by a double-isotope technique. In the lactase-deficient group, the mean absorptions were 33.5% from hydrolyzed milk and 36.2% from the same volume of unhydrolyzed milk (P greater than 0.30). In the lactase-sufficient group, mean absorptions were 24.2% from hydrolyzed milk and 25.7% from unhydrolyzed milk. The mean calcium absorption from both lactose-hydrolyzed milk and unhydrolyzed milk was significantly greater (P less than 0.01) in the lactase-deficient group compared to the lactase-sufficient group, presumably reflecting lower dietary calcium intake in the former. These data indicate that, in lactase-deficient subjects, malabsorption of lactose does not affect calcium absorption.