Nine subjects with different training background were studied by means of muscle biopsy from musculus vastus lateralis. The cross-cut area of the different muscle fibre types was measured. By the use of a soft tissue X-ray technique, computed tomography, a cross-sectional picture of the thigh was produced at the same level as the biopsy was taken. The total cross-cut area of the vastus lateralis muscle was measured from this picture. This technique to measure a cross-cut muscle area from an X-ray picture was evaluated and found to be accurate and reproducible. The mean fibre area was highly correlated (r = 0.91; P less than 0.001) to the cross-cut area of the vastus lateralis muscle. The total number of fibres in the vastus lateralis muscle was estimated. It was concluded that the cross-sectional area of the thigh muscle can be accurately determined by means of computed tomography. The larger cross-sectional area of the vastus lateralis muscle in well trained subjects was primarily explained by a larger cross-sectional area of the fibre, while the total number of fibres in the vastus lateralis muscle seemed to be fairly equal among the subjects.