Twenty-five patients with early breast cancer (T0-T2, N0-N1, M0) have been studied prospectively to determine the relationship between marrow micrometastases, disease-free interval and survival. Marrow specimens were aspirated from three sites immediately prior to breast surgery. An immunocytochemical technique using monoclonal antibody LICR.LON.M8.4 was employed to detect micrometastases. The minimum follow-up was 38 months. Twelve of the 25 patients (48%) had micrometastatic lesions in their marrow at presentation. Four of these patients developed distal recurrence during follow-up, causing death in two of them. Five of the 13 patients with no evidence of micrometastases developed distant recurrence and four of them have died. There was no correlation between the state of the marrow and the development of metastatic disease, although axillary lymph node status, disease stage and tumour volume correlated significantly with outcome (all P less than 0.025). Micrometastatic lesions appear to be common in the marrow of patients with early breast cancer. We have been unable to demonstrate that they have prognostic significance.