This study was designed to investigate whether the serum tumour marker CA 549 gave early and reliable information about disease activity among metastatic breast cancer patients during cytostatic treatment and follow-up. 50 females with metastatic breast cancer were monitored clinically and with the tumour marker CA 549. Response evaluation was based upon clinical (World Health Organization) and elaborated CA 549 criteria, respectively. In 113 blindly and matched evaluations, concordance appeared in 73/113 and discordance in 40/113 evaluations. In 27, discordance concerned degree of response, in 2 clinical progression followed marker progression after the end of the study, and in 11 progressive disease was established by clinical investigation alone. CA 549 response excluded clinical progression in bone or viscera and reversed. Clinical progression within 2 months in viscera and bone was predicted among 91% by marker progression. Clinical progression was excluded among 93% without marker progression. In conclusion, monitoring of metastatic breast cancer patients could include CA 549 if standardised criteria for marker evaluation are used.