Metastatic breast cancer cells were found in the bone marrow of 60 (23%) of 269 patients with primary breast cancer, none of whom had metastatic disease disclosed by any other investigation, including bone scanning and radiological skeletal survey. We estimated the number of cancer cells as less than or more than 20 cancer cells seen. Twenty-six patients had less than 20 cancer cells present, and 34 had 20 or more. At a median follow-up time of 22 months, 53 patients had relapsed, 19 of 60 (31.7%) in the group found to have micrometastases and 34 of 195 (17.2%) in the group that had normal bone marrow. Patients with micrometastases are relapsing at a faster rate than those without micrometastases (P = less than 0.05). Patients with less than 20 cancer cells present are relapsing faster than those with no cancer cells but slower than those with 20 or more cancer cells (P = less than 0.01). We conclude that the presence of cancer cells in the marrow at primary diagnosis is a prognostic factor in patients with primary breast cancer.