Zinc is one of the essential microelements involved in the regulation of enzyme activity, as well as metabolism of nucleic acid and proteins. There have been few reports on equine serum zinc concentrations during the training period, and little is known about the relationship between zinc levels and diseases in horses. In this study, we measured serum zinc levels in healthy Thoroughbred racehorses, as well as in other horses, under general disease or training conditions. The reference value for serum zinc levels in Thoroughbred horses was 41–79 μg/dl. There were no differences in serum zinc levels due to sex or age. Significant decreases in serum zinc levels were observed after training, but serum zinc levels did not vary with intensity of sweating. Serum zinc levels were lower in horses clinically diagnosed as having shipping fever (36.3 ± 2.7 μg/dl), fever (45.3 ± 3.0 μg/dl) and cellulitis (44.0 ± 3.4 μg/dl), as compared to control values (59.7 ± 9.7 μg/dl). They also tended to decrease in experimentally infected horses one day after inoculation. Changes in serum zinc levels reached nadir one day after surgical invasion, except for a horse that experienced complicating shock. These results suggest that zinc is a serological indicator of inflammatory status in Thoroughbred horses.