Accurate determination of skeletal muscle size is important in studies of ageing, disease and exercise physiology. Obtaining serial cross-sectional images using magnetic resonance imaging (MRI) can be considered the “gold standard”, but is expensive and time consuming. Dual-energy x-ray absorptiometry (DXA) is more accessible and is growing in popularity, but DXA does not distinguish individual muscles. It is not known to what extent DXA-determined thigh lean mass (DXAthigh) correlates with MRI-determined quadriceps muscle volume (Qmv) in a diverse subject group. The quadriceps occupies a constant portion (51±5%) of thigh muscle volume (Narici & Roi. 1992), so it is reasonable to expect strong agreement between Qmv and DXAthigh. To investigate this, 10 young (24 ± 1 yrs) and 11 elderly (68 ± 1 yrs) adults (9 female, 12 male), after providing written informed consent, completed an MRI scan (Esaote; 0.25-T; G-scan) and a whole-body DXA scan (Lunar Prodigy, GE Medical) with subjects lying supine. For MRI, serial cross sectional slices were obtained perpendicular to the thigh at 25 mm intervals from the distal to the proximal ends of the femur using a turbo 3D T1-weighted protocol. Quadriceps muscle cross-sectional area was determined in each slice and Qmv estimated by multiplying the cross sectional areas by the slice thickness (2.8 mm) and distance between slices. For DXA, lean mass was determined in the thigh region from the ischeal tuberosity down to the knee joint (upper and lower margins, respectively). Students’ t-tests were used to compare between groups and coefficient of determination used to determine the relationship between variables. Data are mean ± SD. Average Qmv was 1,782 ± 563 cm3 (range: 740 – 2967 cm3). Average DXAthigh was 4.8 ± 1.2 kg (range: 2.7 – 6.9 kg). Compared with elderly, young adults had significantly larger Qmv (1284 ± 313 vs. 2066 ± 502 cm3, respectively; P < 0.001) and significantly larger DXAthigh (4.1 ± 0.8 vs. 5.6 ± 1.2 kg; P = 0.01). A very strong relationship existed between Qmv and DXAthigh (r2 = 0.86; P < 0.001), and from this, Qmv can be estimated from the equation: Qmv = 420.67*DXAthigh – 379.32. These results demonstrate good agreement between the thigh lean mass determined with DXA and quadriceps muscle volume determined with MRI in a diverse group of subjects in a cross-sectional study design. Considering that MRI measures take approximately 90 minutes per subject, while DXA takes only about 20 minutes, is considerably cheaper and provides additional descriptive data such as whole-body lean mass, fat mass and bone mineral density, DXA can be considered a good alternative to MRI for evaluating muscle mass. However, it remains to be determined whether DXA is accurate enough to track longitudinal changes in muscle size, e.g. following exercise training interventions or muscle wasting conditions.