The hydroxyproline/creatinine excretion ratios have been measured in half-hour fasting morning urine specimens from breast cancer patients and compared with corresponding values measured in 24 hour urine collections. Both parameters show a close correlation. The value of the half-hour test in predicting the presence of early bone metastases, in monitoring the response of patients to therapy and in eliminating unwanted effects of dietary Hydroxyproline intake are discussed.