The aim of the present study was to assess the clinical value of five serum tumour markers AFP, CA 15-3, CEA, TPS and Neu oncoprotein (p 185) in the diagnosis of breast cancer. The serum values were measured in a prospective series of patients with breast cancer (n = 233), benign breast disease (n = 176) and healthy control subjects (n = 215). The cut-off levels (90% specificity) were determined for each test. Using these cut-off levels the diagnostic sensitivity of the CEA test was 0.23, for the AFP test it was 0.16, for the CA 15-3 test 0.27, for the TPS test 0.18 and for the Neu oncoprotein test 0.19 in detecting breast cancer. Correlation coefficients (Pearson's) were statistically significant between CEA and CA 15-3, and between CA 15-3 and Neu oncoprotein measurements in breast cancer patients. In patients with benign breast disease the serum levels of AFP, CA 15-3, CEA and TPS correlated with age, being somewhat higher in older patients. Similarly, in population controls higher age correlated with higher levels of AFP, CA 15-3, Neu and TPS. In breast cancer patients there was no correlation between the age at diagnosis and any of the measured markers. The only marker statistically significantly associated with the stage of breast cancer was CA 15-3. In conclusion, our results indicate that serum CEA, AFP, CA 15-3, TPS and Neu oncoprotein are only of limited value in the diagnosis of breast cancer.