58 patients with clinically localised breast carcinoma, treated by either mastectomy alone or lumpectomy plus local radiotherapy, have now been followed for up to 51 months (median 12 months). 21 of the 58 patients (36.2%) had a persistently elevated or rising fibrinopeptide A level after surgery and 11 of these patients (52%) to date have subsequently developed recurrent breast carcinoma up to 27 months after the fibrinopeptide A level became elevated (median 6 months). Conversely, only 2 (5.4%) of the 37 patients with persistently normal post-operative fibrinopeptide A levels have developed recurrent disease. Elevated fibrinopeptide A levels appear to be a marker of persistent tumour activity and precede clinical recurrence in certain patients with breast carcinoma, but normal values do not exclude recurrent or residual disease.