Background Does klotho (KL) protein exist in human serum, and is there any correlation between KL protein in serum with human aging? In order to answer those questions, we identified KL protein in human serum and established the correlation between KL protein in human serum and aging Methods We prepared a polyclonal antibody against human KL protein that was able to recognize the C-terminal of human secreted KL protein Western blot and enzyme-linked immunosorbent assay (ELISA) were used to identify KL protein in human serum Results In Western blot, the antibody specifically recognized a 60-kD KL protein in both human and mice serum The population aged from 0 to 91 years screened by ELISA revealed that the level of serum KL declined while age increased, though each individual level was variable and that the trend of decreasing in serum KL had no difference in sex Conclusion Our data suggest that KL is a serum factor related to human aging Background Does klotho (KL) protein exist in human serum, and is there any correlation between KL protein in serum with human aging? In order to answer those questions, we identify KL protein in human serum and established the correlation between KL protein in human serum and aging Methods We prepared a polyclonal antibody against human KL protein that was able to recognize the C-terminal of human secreted KL protein Western blot and enzyme-linked immunosorbent assay (ELISA) were used to identify KL protein in human serum Results In Western blot , the antibody specifically recognized a 60-kD KL protein in both human and mice serum The population aged from 0 to 91 years screened by ELISA revealed that the level of serum KL declined while age increased, though each individual level was variable and that the trend of decreasing in serum KL had no difference in sex Conclusion Our data suggest that KL is a serum factor related to human aging