To the editor: In October 1985, we described (1) a method for estimating true fractional calcium absorption, which requiredonly a single blood sample taken 5 hours after ingestion of an isotopically labeled oral test load. The algorithm was based empirically on our studies in normal perimenopausal women, in whom true fractional absorption had been measured simultaneously using the rigorous, double-isotope method (2). The coefficient of determination for fractional absorption estimated with this equation, relative to the value obtained with the double isotope method in the same women, was 0.908. Our algorithm was expressed in this equation (1): fx abs =