Calf and quadriceps muscle size and composition have been examined in 5 control subjects and 12 boys (aged 7-18 years) with Duchenne muscular dystrophy using computed tomography (CT) and quantitative histochemistry of needle biopsy samples of muscle. In assessing the size of leg muscles it was found that in control subjects the muscle cross-sectional area was related to the cross-sectional area of the femur. The calf muscles in the patients were enlarged both relative to the quadriceps and absolutely when related to the cross-sectional area of the femur used as an index of stature. The enlargement of the calf was the result of accumulations of fat and connective tissue in the presence of relatively normal (compared to femur size) amounts of muscle tissue. Although hypertrophied muscle fibres were a feature of the biopsy samples in the calf the extent of this was insufficient to contribute appreciably to the total muscle enlargement.