To determine the incidence and clinical significance of micrometastases in the bone marrow of breast carcinoma patients, we performed an immunoalkaline phosphatase assay using anticytokeratin (AE1, AE3, MAK-6) and antiepithelial (113F1, 260F9, 317G5) antibodies on the bone marrow aspirates of 71 stage IV disease patients with either recurrent regional or distant metastases. Although we detected tumor cells within the bone marrow of 38% of these patients with this assay, no significant correlation was seen with patient's age, menopausal status, bone scan, bone marrow core histology, response to induction chemotherapy, number of metastatic sites, dominant site of metastasis, or subsequent clinical outcome. The clinical parameters that were associated with improved survival were one dominant site of metastatic disease and regional soft tissue recurrence without distant disease.