Grey-scale ultrasonography can produce an image of the quadriceps muscle from which it is possible to measure its cross-sectional area (CSA). The between-days coefficient of variation for quadriceps CSA (at mid-thigh) in 14 legs of seven subjects each scanned on four days was reduced to 4.0% by averaging four scans on each day. Bilateral scans (at the mid-thigh level) were used to measure the severity of quadriceps wasting in 21 otherwise healthy adult patients with a difference in thigh circumference following unilateral knee immobilization or injury. Quadriceps wasting as demonstrated by the scans was consistently more severe than the disparity in whole thigh cross-sectional area at the same level or the disparity in anthropometric estimates of fat-free thigh volume. Investigations concerned with changes in quadriceps muscle bulk must therefore use a technique (such as ultrasonography) which allows measurement of the quadriceps itself.