The value of tumour-associated antigens CEA and CA 15-3 was studied in patients with breast cancer over a 4-year period. A total of 252 patients with primary or recurrent disease had available and corresponding CEA and CA 15-3 values at diagnosis and during follow-up and were studied in detail. Preoperative and three-monthly serial postoperative levels were measured in each patient. Ten of 11 patients presenting with primary and concurrent metastatic disease had elevated CA 15-3 levels (> 25 I.U./ml) as compared to 6 with CEA (> 5 ng/ml). Fourty-seven patients developed locoregional recurrence of which 15 had concurrent metastatic disease. CA 15-3 was elevated in 14 cases while CEA in 11. Of 32 patients with locoregional recurrence alone, 18 later developed metastatic disease at a mean follow-up time of 17.5 months. There was a significant correlation between CA 15-3 value at locoregional recurrence and time to subsequent metastasis (r = 0.-0.57, P = 0.0133). CEA was elevated in 64%, CA 15-3 in 87% and either marker in 94% of 87 patients diagnosed with metastatic disease. Of 53 patients with serial markers and metastatic disease, 72% (38/53) had rising CA 15-3 levels prior to diagnosis with a mean lead time of 9.9 months. Use of CEA in conjunction improved lead time detection to 83%. This study demonstrates that CA 15-3 is superior to CEA at detecting metastatic disease at initial presentation and during follow-up. Use of CEA in conjunction with CA 15-3 improves the detection of systemic disease.