After surgery and radiotherapy circulating serum levels of the tumour markers MCA and Ca 15-3 were evaluated in 226 clinically disease-free breast cancer patients. 15 of them presented with local recurrences and/or distant metastases after a follow-up period of twelve months. Six patients belonged to the group of 180 with both markers negative, two belonged to the 32 patients with only one marker positive, and seven to the group of 14 with both markers positive. After twelve months the probability of disease-free survival (calculated by the method of Kaplan-Meier) is 96% in patients with negative tumour markers and only 51.4% in patients with both markers positive (p less than 0.001). After 16 months these values are 96% versus 25.7% with the same statistical significance. The value of tumour marker examinations in the regular follow-up of patients with breast cancer lies in the early detection of tumour activity and therefore in a better chance of response to subsequent treatment. The diagnostic sensitivity of MCA was 75.5%, and for Ca 15-3 70.24%.