Single cells from mammary carcinoma infiltrating bone marrow can be detected in marrow aspirates using immunocytochemical stains for epithelial membrane antigen (EMA). This technique has been used to examine marrow aspirates taken from multiple sites from 24 patients at surgery for breast cancer. Ten of these patients had EMA-positive cells in their marrow, while 32 marrow samples from patients who did not have carcinoma were negative. These results have been combined with those obtained by taking aspirates from single sites from 47 breast patients without known skeletal deposits. Follow up showed that the patients with EMA-positive cells in their marrow developed bone metastases at a significantly faster rate.