An essential part of the classification of prostate carcinoma is the diagnosis of bone metastases. This was done with 70 patients using x-ray analysis, scintography, determination of the acid and alkaline phosphatase, and pelvic crest biopsy, as well as aspiration of the pelvic and sternal bone marrow. In addition, the hydroxyproline concentration was determined in the 24-hour-urine. The study, which was initially undertaken on a sample group (n = 145), yielded a high correlation between age and sex and hydroxyproline values. Women before menopause show significantly lower values than do men of the same age. The data on patients with prostata cancer (n = 70) showed that patients with and without bone metastases, who had been treated with estrogens, had a significantly lower quantity of hydroxyproline than did patients who had not received estrogen therapy. Patients with skeletal metastases (n = 24) showed significantly higher hydroxyproline excretion in the urine than did those with prostate cancer without metastases, or healthy men of the same age (n = 35). Comparison of the results of hydroxyproline determination with the other diagnostic methods for demonstrating bone metastases showed that hydroxyproline determination was diagnostically on par with the scintigram. Pelvic crest biopsy, pelvic and sternal marrow aspiration can be considered valuable supplementary diagnostic procedures.