Bone marrow biopsies (BMB) were performed on 622 women with a history of primary or metastatic breast cancer. In 332 of the patients bone scintigraphy (BSC) was also carried out at the time of initial diagnosis or during follow-up. The BMB revealed metastases in 22/88 patients (25%) in Stage I--III with positive BSC and in 39/111 patients (35%) in Stage IV with positive BSC, a total of 61/199 patients (31%). Moreover, the BMB was positive in 6/73 patients in Stage I--III with negative BSC, and in 5/37 patients in Stage IV with negative BSC, a total of 11/110 patients (10%). Statistical evaluation showed that the patients with BMB revealing tumor had a significantly higher death risk (P less than 0.0000001) than patients with BMB revealing tumor had a significantly higher death risk (P less than 0.0000001) than patients with negative BMB. No significant correlation was found in patients with positive or negative BSC. Similar results were also found for a subgroup of 207 patients who underwent BSC and BMB within five months. BMB may therefore be indicated for verification of metastases and prediction of prognosis in all patients with positive BSC and in BSC-negative patients Stage II--IV.