The values of hydroxyprolinuria were determined in 314 cases of Paget's disease and 91 cases of osteoma. During Paget's disease, the values found were related to the degree of the bony disorder. A more detailed study of the hydroxyproline polypeptides was undertaken, comparatively in a few cases of these two diseases. In Paget's disease, there was shown a relative increase in the oligopeptide fraction, whereas the polypeptide fraction was increased in osteoma. By chromatographic and electrophoretic technics, it was shown that the acid oligopeptides (containing mainly the dipeptide (Glu-Hyp) are in percentage, reduced in Paget's disease and increased in osteoma, whereas the neutral oligopeptides (containing mainly Pro-Hyp) undergo the opposite variation in these two diseases.