Altogether 15 partially independent measurements of bone mass in 100 women with clinical and roentgenological signs of osteoporosis were correlated to the alkaline phosphatase activities of the same individuals. There was a slight but significant negative correlation indicating an increasing alkaline phosphatase activity with decreasing bone mass. This correlation was not caused by interaction of age. There was no correlation or morphological signs of osteomalacia. The changes could not be explained by fractures. It is suggested that a slight increase in the alkaline phosphatase activity in women with a more severe osteoporosis is related to bone resorption.