Serum levels of CA 15-3 were determined in normal women and patients with breast cancer and other diseases. The serum concentration of normal subjects was 9.8 +/- 4.4 units/ml (mean +/- S.D., n = 97). The patients with primary and recurrent breast cancer had significantly higher levels of CA 15-3, their values being 23.1 +/- 59.6 (n = 47) and 127.6 +/- 179.7 (n = 32) units/ml, respectively. When the value of 20 units/ml was used as the cutoff for the normal value, the positive rates were 4%, 26% and 75% in normal women and breast cancer patients primary and recurrent, respectively. No increased levels were found in the sera of patients with benign breast diseases or with gastrointestinal, thyroid, lung and other cancers. The elevated levels of CA 15-3 decreased after successful therapy both in primary and recurrent breast cancer patients, while there was no change in nonresponders. The measurement of serum CA 15-3 would be useful for monitoring the therapy and for detection of the recurrence of breast cancer.