A novel immunoassay specific for the osteoclast-produced TRAP isoform 5b has been developed recently. By means of this assay we studied the usefulness of serum TRAP-5b in monitoring the response to palliative treatment with pamidronate in breast cancer patients with bone metastases. We correlated serum TRAP-5b levels with pain intensity and intake of analgesics to assess the possible utility of the marker in identifying patients who could benefit from pamidronate treatment. Twenty-eight advanced breast cancer patients with bone metastases entered the study. Patients were treated according to the following schedule: two two-week cycles of 60 mg/week pamidronate IV, with a three-week interval in between (six infusions over seven weeks), followed by one infusion every three weeks for a total of 24 infusions over a treatment period of 61 weeks. Blood samples were taken before the start of treatment and before each infusion during two treatment cycles. To measure serum TRAP levels we employed the new immunoassay kit BoneTRAP produced by Suomen Bioanalytiikka Oy (SBA), Oulu, Finland. In order to assess the usefulness of this marker in evaluating the response to pamidronate treatment we divided patients into two groups (group A, worsened; group B, improved) with respect to pain trend and analgesic intake. Our results did not show any statistically significant difference in baseline serum TRAP levels in the two groups. However, one week after the first pamidronate infusion TRAP-5b serum levels decreased by 39% and 18% in groups A and B, respectively (p=0.01); these levels persisted throughout the treatment period. In conclusion, a decrease in TRAP-5b serum levels may reflect the pharmacological activity of pamidronate and seems to predict pain relief and a reduction in analgesic consumption.