Leptin and apoptosis inhibitor soluble Fas antigen in the serum of patients with osteosarcoma and neuroectodermal bone tumors. Serum contents of leptin and soluble Fas antigen were measured in 28 patients with osteosarcoma, 7 with neuroectodermal bone tumors, and 17 healthy subjects. The incidence and levels of soluble Fas antigen in patients with osteosarcoma and neuroectodermal bone tumors were higher than in healthy subjects and did not depend on sex and age in both healthy subjects and patients. Serum concentration of leptin in women was higher than in men (both in healthy controls and patients) and was lower in healthy subjects compared to patients with osteosarcoma and neuroectodermal bone tumors. A trend to a negative correlation between the concentrations of leptin and soluble Fas antigen in female patients with osteosarcoma and male patients with neuroectodermal bone tumors was revealed. The role of leptin and soluble Fas antigen in the pathogenesis of primary bone tumors is discussed.