With use of 99mtechnetium-labeled phosphates, we performed preoperative bone scans on 122 women with biopsy-proved breast carcinoma. Only two of the 110 patients with Stage I or II disease had scan abnormalities interpreted as bone metastases. Of 55 patients with normal preoperative scans, 20 later had changes suggesting bone metastases on the subsequent scans, most within 24 months of operation. In women with Stage I and II tumors, 13 of 48 (27 per cent) had scan evidence of bone metastasis of postoperative follow-up examination. Five of 23 with potential surgical cures (negative lymph nodes at operation) had bone metastasis within two years of operation. In Stage I and II patients, postoperative discovery of evolving metastases was most often (11 to 13) made by bone scan. Although the initial yield from preoperative bone scans is low, preoperative scanning combined with sequential postoperative scans constitutes one of the most sensitive indicators of evolving metastatic disease.