S-phase cells of 66 primary breast cancers were labeled in vivo by preoperative infusion of the thymidine analogue bromodeoxyuridine. A monoclonal antibody specific for DNA-incorporated bromodeoxyuridine was used to identify positive cells and compute a labeling index on histologic sections. The labeling index (the percentage of cells in S-phase) ranged from 0.1% to 23.9%; it correlated positively with poorly differentiated cancer, higher mitotic counts on routine histologic examination, and tumor size; and it correlated inversely with estrogen and progesterone receptors. The labeling index did not correlate with nodal involvement or ploidy. Of the 15 patients with a labeling index greater than 12%, three died and one had systemic disease after a median follow-up of 19 months. No other patients had recurrences. There were no clinical complications of bromodeoxyuridine infusion.