We developed an enzyme-linked immunosorbent assay (ELISA) for vascular endothelial growth factor/vascular permeability factor (VEGF/VPF). The assay revealed that VEGF/VPF levels in the sera of mice were significantly increased from undetectable level by s.c. transplantation with a solid tumor. We also measured VEGF/VPF levels in serum specimens obtained from cancer patients with several types of cancers. VEGF/VPF levels in the sera from cancer patients were significantly higher than those in the sera from the individuals with no sign of cancer.