It has been gradually recognised that urinary hydroxyproline might be a good index of collagen break-down or turnover, but the details have not been completely clarified. The following three points are discussed in this report; Variations of urinary total hydroxyproline excretion (1) in the human aging process. (2) in patients with growth disturbance of bone, and (3 ) in pati ents with malignant bone tumor, with observations made on the urine specimens of 134 normal healthy subjects consisting of 73 males and 61 females and their values classified by age and sex. The highest excretion was shown in the group of 10 to 14 years of age, with statistical significances noted between the highest excre tion group a nd that of 6 to 9 years age group, as well as between the former a nd the group of 15 to 19 years of age. In patients with systemic bone disease lower excretion was observed in pituitary dwar fism, hypothyroidism, osteogenes is imperfect a. However, in cases with positively advanced bone age, excretion was elevated as in adrenogenital syndrom e and pituitary dwarfism during therapy . It has been proved that there occurs an extremely increased excretion of hydro xyproline in patients with malignant bone tumor, especially in osteogenic sarcoma, and the changes in excretion were observed to parallel the clinical progress of the pat ient after surgical interven tion. In the ex perimental study using Walker carcinosarcoma 256 urinary excret ion mark edly increased with aggravation of bone lesion a nd after amputation no e levated excretion was observed. As reported in this paper the c hange in urinary hydroxyproline could b e a good index of bone collagen metabolism, showing elevated excr etion in growing children, decrease in patients with hormonal growth disturbances and alteration in those with bone tumor or systemic bone disease.