The urinary excretion of Hydroxyproline (OH-P), evaluated as OH-P/creatinine ratio, has been examined in the follow up of 142 breast cancer patients. The mean value obtained in the women with bone lytic lesions, prior to treatment, was 5.3 +/- 1.9 (min. 3, max. 9), whereas in the women without bone metastases it was 1.7 +/- 0.5 (min. 0.6, max. 3), similar to the control group. The patients affected by osteoplastic lesions had a normal OH-P/creatinine ratio. The highest values of OH-P/creatinine ratio were found in women with bone marrow infiltration, ascertained by bone needle biopsy. In the group of patients with bone lytic metastases, a good correlation between changes of OH-P/creatinine ratio and response to hormonal or chemotherapeutic treatment has been observed. Therefore the OH-P/creatinine ratio could be effectively useful, with other clinical parameters, in the follow up of breast cancer patients.