Metabolic markers of bone metabolism may be useful for the diagnosis and monitoring of bone metastasis in breast cancer patients. Serum tartrate-resistant acid phosphatase 5b (TRACP5b) activity is a novel bone resorption marker. The treatment response of serum TRACP5b activity, bone alkaline phosphatase (BAP) activity, and concentrations of NH(2)-terminal telopeptide of type 1 collagen (NTX) in 68 breast cancer patients with bone metastasis were determined. These patients were treated and followed up as clinically indicated. Fifty-four healthy women were recruited as control. Serum TRACP5b activity, BAP activity, and NTX level of breast cancer patients with bone metastasis were significantly higher than those of normal controls. In normal subjects, serum TRACP5b activity and NTX level are significantly correlated (P < 0.0001). Neither was correlated with BAP activity. In breast cancer patients with bone metastasis, all marker pairs correlated to each other significantly (P < 0.0001). Biomarkers were examined repeatedly in 38 patients who were evaluable for treatment response. Based on clinical criteria, 20 patients were responders and 18 were nonresponders. In the 20 responders, serum TRACP5b activity and NTX level decreased significantly (P < 0.0001 and 0.0107, respectively) after treatment. In the 18 nonresponders, only NTX level showed significant increase (P = 0.0342) after treatment; TRACP5b and BAP were unchanged. By means of multiple logistic regression with stepwise selection, we determined that TRACP5b activity has a higher probability than NTX level to indicate treatment response as a function of percent change after treatment (18 times versus 12 times). Our data support the use of either TRACP5b activity or NTX level to follow up breast cancer patients with bone metastasis after treatment instead of the prevailing BAP activity.