To measure the motions of the trapeziometacarpal joint of the thumb quantitatively, a roentgenographic method was developed and tested using T-shaped metal markers, a special cassette-holder, and biplane roentgenograms. Two experiments were performed. In the first one, the metal markers were fixed to the trapezium and third metacarpal in ten cadaver specimens, and a fixed spatial relationship between the trapezium and the third metacarpal was identified roentgenographically. This relationship was that the reference axes of the trapezium were aligned at median angles of 48 degrees of flexion, 38 degrees of abduction, and 80 degrees of pronation with reference to the reference axes of the third metacarpal. In the second experiment, in the dominant hand of nine male and ten female subjects (average age, twenty-six years) T-shaped markers were fixed to the skin overlying the third metacarpal and the metacarpal and phalanges of the thumb. Using the same roentgenographic technique and coordinate systems employed in the first study, the average total motions of the trapeziometacarpal joint (determined as motions of the first metacarpal with reference to the third metacarpal) were 53 degrees of flexion-extension, 42 degrees of abduction-adduction, and 17 degrees of axial rotation (pronation-supination). In addition, six functional positions of the thumb were studied: rest, flexion, extension, abduction, tip pinch, and grasp. A position of adduction and flexion of the trapeziometacarpal joint was most common during thumb function, and both the trapeziometacarpal and metacarpophalangeal joints contributed to rotation of the thumb.