Serum activity of tartrate-resistant acid phosphatase 5b (TRAP 5b) in patients with breast cancer and prostate cancer having bone metastases was much higher than in healthy donors and patients without skeletal injuries. TRAP 5b activity in patients with breast cancer and multiple bone metastases surpassed that in patients with single bone metastases. The mean activity of TRAP 5b and range of enzyme activity in women treated with bisphosphonates were significantly lower than in patients not receiving antiresorptive therapy. Diagnostic sensitivity and specificity of TRAP 5b as a marker of skeletal metastases in patients with breast cancer were 82 and 87%, respectively. In patients with prostate cancer these indexes were 71 and 83.4%, respectively. Detection of this marker in tumor patients holds much promise for early diagnostics of bone metastases, estimation of the severity of skeletal metastases, and monitoring of the efficiency of bisphosphonate therapy.