The value of bone marrow biopsy (BMB) in advanced breast cancer at the time of first relapse was studied in a prospective manner. Bone marrow biopsy was performed in 142 consecutive unselected metastatic patients: 129 at the time of first recurrence, and 13 in patients with metastases at the time of first diagnosis. Overall, BMB was positive in 32 patients (23%). In the group with negative bone x-ray, it was positive in two patients of 84 (2%); both of them had doubtful scan. In the group with positive x-ray, BMB resulted positive in 30 of 58 (52%). There was a significant correlation between number of bone segments radiologically involved and BMB positivity rate, ranging from 15% in the patients with only one, to 68% in those with more than three sites involved (P = 0.02). Patients with x-ray evidence of metastases in the pelvis had significantly higher rate of BMB positivity (67% versus 32%; P = 0.02).The median survival time from the first relapse was 153 weeks in BMB-negative cases and 149 in positive ones. Considering only the patients with demonstration of bone invasion obtained with either or both x-ray and BMB, 34/62 patients had positive BMB (55%). In these cases BMB was found more often positive in patients 50 years or younger than in patients older than 50 years (80% versus 47%; P = 0.05); the median survival time was longer, but not significantly, in BMB-positive patients than in negative ones (149 weeks versus 119; P = 0.3). The authors conclude that BMB is not required in common restaging procedure when both bone survey and scan are negative. Bone marrow biopsy results are more often positive in younger patients and survival is not negatively affected by bone marrow invasion as diagnosed by BMB.Cuncer 61:1041-1045,1988.HE CHOICE OF a proper therapeutic approach of T breast carcinoma is based on an accurate staging procedure, both at time of initial diagnosis and at time of first relapse. The presence or absence of distant metastases is the main prognostic factor, as advanced disease, currently has no chance of cure.The skeleton is a very frequent site of metastatic spread with a reported figure of 50% at the time of first relapse and of 50% to 70% at autopsy.'S2 Bone marrow too, is one of the most frequent disease sites. To some authors, skeletal metastases are always preceded by bone marrow inva~ion.~ Bone marrow biopsy (BMB) already is routinely used.From the