Bone marrow biopsies (BMB) were performed in 173 consecutive unselected breast cancer patients at the time of diagnosis to define the value of this diagnostic tool in the initial staging of mammary carcinoma. In a group of 160 patients with a negative standard staging work-up, BMB was positive in two (1%). Both of them had negative x-ray but bone scan was positive in one and doubtful in the other. Bone marrow biopsy was positive in 31% of 13 additional patients with metastatic disease and in 44% of the nine among them with radiologically involved skeleton. These results exclude that BMB is able to discover micrometastatic foci of neoplastic disease. Its positivity appears strictly correlated with that of bone x-ray and scan. Based on the results of this prospective study, BMB is not required when both bone survey and scan are negative, but could be useful in clarifying diagnostic doubts of skeletal involvement.