Klotho, a protein with anti-aging properties, regulates phosphate, calcium and bonemetabolism, induces resistance against oxidative stress and may participate in insulinsignalling. Neonates, especially preterm ones, are prone to metabolic disturbances andoxidative stress. The role of kltho in neonates is not known.The objective of this prospective study was to evaluate circulating klothoconcentrations in preterm and full-term neonates and unravel possible correlationswith growth, metabolism and indices of oxidative stress.Plasma Klotho levels were determined by specific ELISA in 50 healthy neonates (25preterm, mean [SD] gestational age 33.7 [1.1] weeks and 25 full-term infants) at days14 and 28 of life. Associations of Klotho with anthropometric (body weight andlength) and metabolic parameters (serum calcium, phosphate, 1,25-dihydroxy-vitaminD, parathormone, FGF23, glucose, insulin, homeostasis model assessment index ofinsulin resistance [HOMA-IR]), and indices of oxidative stress (malonyldialdehyde[MDA] concentration and superoxide dismutase activity) were examined.Statistical analysis led to the following results:1. Plasma Klotho levels are significantly higher in full-term than preterm infantsat both days 14 (1099  pg/mL vs. 884  pg/mL, respectively; P<0.05) and 28(1277  pg/mL vs. 983  pg/mL, respectively; P<0.01).2. In both preterm and full-term infants, Klotho levels increase significantly fromday 14 to 28 of life (P<0.001).3. Circulating -Klotho concentrations correlate with gestational age (r=0.32,P=0.001), body weight (r=0.34, p=0.001), body length (r=0.33, P=0.001), 1,25-dihydroxy-vitamin D level (r=0.24, P<0.05) and MDA level (r=0.20, P<0.05), but notwith glucose, insulin, or HOMA-IR values.In conclusion, plasma klotho levels rise as gestational and postnatal age advance inneonates and may have an impact on vitamin D metabolism and oxidative stress.Whether klotho may have a role in the regulation of infants’ growth needs to befurther studied.