The tumour marker CA 15-3 has been assayed in 130 patients with breast cancer and correlated with stage of their disease at presentation. The median value of CA 15-3 (43 kU/l) in 26 patients with stage IV disease was significantly higher than the median for 97 patients classified as stage I or II (17 kU/l). Values were elevated in 18 of 21 (86%) patients with bone metastases at presentation. For the 41 patients with stage I or II disease presenting with levels of CA 15-3 of greater than 20 kU/l, the disease-free interval and survival were significantly less than for 56 patients presenting with levels of less than 20 kU/l. CA 15-3 provides additional information to conventional staging tests for patients presenting with breast cancer and may also have a role as a prognostic indicator. This may be particularly useful in the selection of patients for neoadjuvant therapy.