Sialic acid constituent of the carbohydrate side chain of glycoprotein hormones, has been studied in the sera of patients with various diseases. Recent studies have indicated that sialic acid has important roles in gonadal functions. However, little is known about serum sialic acid levels in untreated hypogonadal men. To gain further insight into the biological role of sialic acid in male hypogonadism, we determined the levels of total sialic acid with the enzymatic assay method in the sera of 36 hypogonadal men with idiopathic hypoganadotropic hypogonadism (IHH) and 20 age-matched normal men. Total sialic acid levels were found to be significantly elevated in the sera of the hypogonadal men (4.31±1.04 mmol/L, P