Currently, a submaximal protocol is used to measure cardiorespiratory fitness in firefighters by estimating their true aerobic capacity (V[Combining Dot Above]O2max); however, this submaximal test has not been cross-validated among firefighters. Thirty firefighters (85% male, 15% female), completed the submaximal protocol and the maximal (Bruce) treadmill protocol on separate occasions. Pearson's correlation analyses between the submaximal and Bruce protocol revealed a significant moderate positive correlation (r = 0.635, p = 0.005). The range of mean V[Combining Dot Above]O2max values and SDs produced from the submaximal and maximal protocols varied greatly (35.4-50.9 vs. 28.6-58.4 ml·kg·min, and SD of 3.91 vs. 7.22, respectively). The submaximal V[Combining Dot Above]O2 test underestimated the true V[Combining Dot Above]O2max in the majority of firefighters (72.4%) and overestimated the true V[Combining Dot Above]O2max in the remainder of firefighters. Participants with a higher body fat percentage were more likely (p = 0.0157) to have an overestimated true V[Combining Dot Above]O2max than those with a lower-body fat percentage. Our results indicate the current submaximal V[Combining Dot Above]O2 test used to measure cardiorespiratory fitness in firefighters is an improvement over previous protocols. However, our findings also show that the accuracy of this submaximal test for predicting the true V[Combining Dot Above]O2max in firefighters is questionable, and may not identify firefighters who possess substandard cardiorespiratory fitness, particularly in those with a higher percentage of body fat. Thus, the results of this study indicate that improvements to the current Fire Service Joint Management, Wellness & Fitness Initiative (WFI) V[Combining Dot Above]O2 assessment is still needed to accurately reflect the true V[Combining Dot Above]O2max of individual firefighters.