Climbing stairs has previously been shown to be one of the most relevant firefighter tasks. Furthermore, ultrasound (US) assessments of muscle size and quality have been shown to have significant influence on lower extremity performance. The purpose of this study was to examine the influence of vastus lateralis (VL) size and quality on stair climb performance (SCP). Panoramic US images of the VL were obtained to determined cross-sectional area (CSA) and echo intensity (EI) in 49 overweight and obese firefighters. A timed and weighted SCP assessment was performed upon completion of the US assessment. Our results demonstrated that EI (r=0.377, p=0.008), but not CSA (r=-0.031 p=0.833), was related to SCP. When accounting for age and VO2peak, the EI and SCP relationship was attenuated (r=0.237, p=0.108). Additionally, EI was the only significant predictor of SCP. These findings indicate that lower extremity muscle quality, but not size, may contribute to firefighter SCP.