Bone marrow biopsies by Jamshidi needle were performed in 106 breast cancer female patients. Sixty-four of them were in follow-up after mastectomy, and neoplastic involvement of marrow was found in 21 patients (32.8%). Among the 42 women undergoing staging before mastectomy, the incidence of marrow involvement was 11.9% (5 women, all with radiographic positivity). Of the 37 women, either in follow-up or in the staging phase, with bone metastases detected by roentgenographic and isotopic examination, the bone biopsy was positive in 23 (62.1%), and 7 histologically had micrometastases. Three women, without any radiographic or isotopic sign of metastases, had positive biopsies. A good correlation was found between the hydroxyproline:creatinine ratio and neoplastic involvement of bone marrow.