Kay and, independently, Roberts were the first to point out that the phosphatase activity of the blood is increased in osteitis deformans. This observation has been confirmed and extended by subsequent investigators- whose conclusions, based upon determinations on a combined total of approximately 200 cases may be summarized as follows: 1. Increased serum (plasma) phosphatase activity is a consistent manifestation of polyostotic Paget's disease.- In localized Paget's disease, the serum phosphatase activity is usually increased- but the increase may not be marked,, , , and in occasional cases, the values may be within normal limits., , 2. There is a rough proportionality between the degree of increase in blood phosphatase activity and the extent of skeletal involvement, as shown by roentgenologic examination.- In advanced cases, the serum phosphatase may be 100 Bodansky units per 100 cc. or more, as compared with the normal range of 1 to 4 Bodansky units. 3. Discrepancies in this relation may occur, however, particularly in the moderately advanced polyostotic group., , , Disproportionately low values may be encountered, usually in cases clinically quiescent., , Some patients exhibiting disproportionately high values have been followed long enough to demonstrate unusually rapid progress of the disease in their cases. It would appear, therefore, that the level of serum phosphatase activity may reflect the rate of development of the disease in some instances, and that, in this sense, the determination may be of value in prognosis.