Serum beta-HCG and CA-125 as tumor markers in a patient with osteosarcoma: case report. Elevated beta-HCG serum levels are usually an indication of pregnancy or pregnancy-related disorders, but beta-HCG can also be elevated in testis and germ cell tumors. HCG expression by osteosarcoma is a rare phenomenon, with a few documented cases. CA-125 is commonly used to monitor disease progression and treatment response in ovarian cancer. CA-125 expression in patients with osteosarcoma has not previously been documented.Elevated beta-HCG and CA-125 serum levels were observed in a female patient of 57 years of age with metastatic osteosarcoma during screening investigations prior to participation in a phase I clinical trial. Pregnancy was excluded. Immunohistochemical studies revealed the tumor to be the source of the elevated beta-HCG serum levels. We found no CA-125 expression in tumor tissue. The patient was treated with E7080, a novel oral multi-targeted tyrosine kinase inhibitor. We measured serum beta-HCG and CA-125 to monitor treatment response. She had a significant clinical and radiological response after two cycles of treatment, but developed progressive disease after the third cycle. The beta-HCG serum levels seemed to better reflect her disease status than those of the other tumor marker, CA-125.When elevated, beta-HCG serum levels in patients with osteosarcoma might be used to monitor treatment. Treatment of advanced osteosarcoma with tyrosine kinase inhibitors, including E7080, warrants further investigation.