Serum CA15-3, CEA, and BCA225 concentrations were determined in 98 patients with advanced or recurrent breast cancer in an attempt to correlate elevation with clinical status. The rate of serum positivity was 68.4% (67/98), 55.1% (54/98), and 43.9% (43/98) for CA15-3, CEA, and BCA225, respectively. After a 4 weeks-interval, a 20% change of tumor marker concentration from the preceding assay correlated significantly with clinical findings. Significant elevation was predictive of new recurrence or tumor regrowth after complete remission, especially in patients with bone metastasis. The 20% change in concentration at 4 weeks was also useful in patients with tumor marker concentrations persistently beneath the cut-off level for positive. Serological evaluation of tumor markers in patients with advanced or recurrent breast cancer should seek to document 20% changes over a 4 week interval.