This study explores the relationship between the speed of sound (SOS) traveling along the cortex of the third metacarpus (McIII) of actively racing Thoroughbreds with the accumulated workload. Workload was calculated by a total cumulative work index (CWItotal) and by three speed-associated CWIs for the different gaits (trot vs. gallop vs. racing) used during training and racing. The effect of this association on the development of dorsal metacarpal disease and McIII failure was also investigated. The SOS was followed during the 2007 race meet, and compared with the CWI using regression analysis. SOS was significantly associated with CWItotal at 8 of the 9 measured sites, and with all of the sites when using the gait-related CWIs (P values <0.05). The rate of metacarpal SOS increase with CWT total was significantly different at one site in horses presenting signs of metacarpal bone failure; however, when gait-related CWIs were considered 5 SOS measurement sites were significantly different. These results indicate that SOS follows a constant pattern of progression as workload accumulates, and that it may be able to identify horses at risk of bone failure.