Urinary hydroxyproline measurements were performed in a group of health volunteers as well as patients with cancer and myelofibrosis. Patients in whom there was no metastatic involvement of bone marrow excreted an amount of hydroxyproline not different from that of the control group. Those who had marrow metastasis produced elevated levels of hydroxyproline; the highest excretions were observed when marrow fibrosis was associated with metastasis. These results contrasted with those observed in agnogenic myeloid metaplasia patients whose excretions were equivalent to the control group. The result suggests differences in the pathogenesis of myelofibrosis and a technique potentially useful for distinguishing between patients who may otherwise be diagnostic problems.