The assessment of changes in skeletal muscle mass is an important requirement, as several medical conditions involve severe muscle loss (cachexia, sarcopenia) (Heymsfield et al. 2014). Furthermore, accurate measurement of anatomical cross sectional area (ACSA) and muscle volume enable the estimation of hypertrophy produced by different regimes of overloading (Narici et al. 1996, 2003). Computer Tomography (CT), Dual-Energy X-Ray absorptiometry (DXA) and Magnetic Resonance Imaging (MRI) are gold-standard techniques used in clinical settings for the evaluation of muscle mass: yet these techniques are expensive and often not easily accessible. Recently, the use of extended field of view (EFOV) ultrasound has been advocated as a reliable alternative for assessing vastus lateralis and quadriceps ACSA (Ahtiainen et al. 2010, Noorkoiv et al. 2010). However, the validity of the EFOV technique as compared to MRI in the assessment of regional and total muscle hypertrophy in response to chronic overloading regimes has not yet been investigated. The present study aims to determine whether EFOV is an accurate and reliable method for measuring quadriceps femoris (QF) and vastus lateralis (VL) muscle ACSA and total volume, as well as its ability to detect changes thereof during long term overloading regimes. Muscle mass of nine recreationally active males (8 young, 25 ±6 years old and 1 elderly, 65 years old), taking part in an 8-wk resistance training study, was assessed by EFOV US and MRI at 0, 4 and 8 week time points. ACSA was measured at 10, 20, 30, 40, 50 and 60% of femur length (FL) (from the medial patella border to the greater trochanter process) by EFOV US and compared with MRI. Pearson correlation and linear regression between the two techniques showed strong correlation between ACSA at mid regions (40, 50, 60%) of the QF 50 and 60% FL (R2= 0.95 and 0.90, respectively) and the VL (50% and 60% LF (R2= 0.91 and 0.89 respectively). Additionally, the two techniques showed good agreement when the average difference (D) in ACSA was assessed at these specific locations for quadriceps (D = 5.7 and 5.6% respectively) and for VL (D = 18.2 and 17.9 respectively). Furthermore, QF volume and VL volume (up to 60% of FL) showed a strong correlation (R2= 0.85 and 0.89 respectively), as did measurements for whole QF volume (R2= 0.92) (D= 9.3, 20.7 and 11.4% respectively). Despite a systematic underestimation by US, a strong correlation existed between EFOV US and MRI for the values of ACSA and of VOL measured at the three time points (0, 4 and 8 wks). Thus, these results show that EFOV US is a valid and reliable technique to quantify muscle ACSA and volume, and represents a useful tool for the assessment of regional and total changes in skeletal muscle mass.