This study was conducted for evaluation of sialic acid concentration in serum of 19 foals with Rhodococcosis and compared with 18 normal foals in horse stables in Tabriz area in Iran. Sick foals were detected using clinical and laboratory (culture of nasal discharge) findings. Blood samples were collected from jugular vein in all foals and serum was separated. Sialic acid (serum total sialic acid, lipid-bound sialic acid and protein-bound sialic acid) concentrations in the serum were measured in all samples. Increasing of total sialic acid, lipid-bound sialic acid and protein-bound sialic acid in the sick group was significant (p=0.000, p=0.012 and p=0.001, respectively). In the normal group, the correlation between serumic parameters was not significant. In the sick group, the correlation between total sialic acid and protein-bound sialic acid was not significant (r=0.095) but there was a significant correlation between total sialic acid and lipid-bound sialic acid (r=0.842). The correlation between lipid-bound and protein-bound sialic acid was also insignificant (r=0.196). The results obtained revealed that Rhodococcosis increases the serumic concentration of sialic acid and there is a correlation between increases in serumic concentration of various parameters.