The bone marrow of 307 patients with primary breast cancer was examined for tumour cells by immunocytochemistry using an antiserum to epithelial membrane antigen. Micrometastases were found in 81 cases (26.4%) and their presence was related to various poor prognostic factors: spread to lymph nodes, vascular invasion, T stage, and pathological size. The median duration of follow up was 28 months. Seventy five patients relapsed, 60 at distant sites. Of these 60 patients, 26 had micrometastases detected at presentation and 34 were free of micrometastases initially. The relapse free interval was significantly shorter for patients with micrometastases, and these patients had a shorter survival. Analysis of the sites of relapse showed that the test predicted bone metastases only. Thus 10 out of 19 patients (53%) who developed bone metastases at first relapse had micrometastases at presentation compared with only 41 out of 288 patients (14%) who remained free of bone metastases or relapsed in non-skeletal sites. The presence of micrometastases detected at the time of initial surgery in a patient with primary breast cancer is a useful predictor of early relapse in bone and may help in selecting patients for subsequent systemic treatment.