Inorganic pyrophosphate can be accurately determined in 2 to 4 ml of serum or urine by an ion-exchange method. Pyrophosphate degradation by serum alkaline phosphatase and pyrophosphatase is stopped by adding to the serum 0.1 mmol of dithiothreitol (Cleland's reagent) per liter. This method is suitable for screening large numbers of patients. All of the patients studied who had classical symptoms of active osteogenesis imperfecta also had abnormally high concentrations of pyrophosphate in their serum. Of this group, 62% excreted increased amounts of pyrophosphates in their urine. Seven adults with inactive osteogenesis imperfecta had normal concentrations of inorganic pyrophosphate in their serum, but their urinary excretion of pyrophosphate was abnormally great. Patients undergoing therapy with magnesium oxide or calcitonin had normal values for serum and urine pyrophosphate. Determination of inorganic pyrophosphate concentrations in serum may be helpful in studying the progress of this disease and in monitoring therapy.