Bone density and strength are important for the long-term health and soundness of performance horses. Marine-derived mineral complexes improve bone density and strength in laboratory animals and humans. Additionally, nutrients without established requirements in horses, such as boron, silicon and vitamin C, may affect bone health. This study evaluated the effect of feeding a supplement containing a marine-derived mineral complex (BMC™) combined with trace minerals and vitamins (Triacton™) (TA) on bone density in Thoroughbred racehorses. Fourteen horses (average age = 3.1 ± .4 y) were used in a 12-week training study. All horses were fed ad-libitum timothy hay and a fortified concentrate (4.6 ±.5 kg/d) along with 60 g of a commercial electrolyte and free-choice salt. The basal ration supplied levels of nutrients that met or exceeded NRC (2007) requirements for horses in heavy work. The horses were paired by age and randomly assigned to treatments. Seven horses received 120 g/d of Triacton supplement and seven horses received 120 g/day of a placebo pellet. One horse from the placebo group was removed from the study because of lameness unrelated to bone development and has not been included in study results. There was no difference in average age between the two groups. Training consisted of jogging, galloping and breezing. There was no difference in training intensity or duration between the two treatment groups. Radiographs of the left front cannon bone were taken from a dorsal-palmar and a lateral-medial view at 0, 4 and 12 weeks of the study. An aluminum step-wedge was placed in plane with the cannon bone to use as an external measure of bone density. Radiographic photodensitometry was used to measure the density of the lateral, medial, dorsal and palmar cortices of the cannon bone. Differences due to training duration and supplementation were determined using a 2-way ANOVA. Over the 12-week training period, the horses supplemented with TA increased dorsal and palmar cortical bone density (p<.05). Bone density of control horses was not affected by training. Medial and lateral cortical bone densities were not affected by treatment. This study demonstrated that bone density may be positively affected by nutrient intakes above NRC recommended levels or by inclusion of nutrients, such as boron, silicon or vitamin C, which do not have established requirements in horses.