Using a three-dimensional analysis, the internal forces in the joints and soft tissues of the thumb during pinch and grasp were calculated. To do this, mechanical equivalents were ascribed to the anatomical system, the joint orientation and tendon locations were determined from biplanar roentgenograms of five marked normal cadaver specimens, and the magnitudes of forces in the tendons, intrinsic muscles, joint contact surfaces, and constraining ligaments were calculated based on assumed loads applied to the tip of the thumb in various types of pinch and grasp. These results are the direct extension of a two-dimensional analysis that proved inadequate for the determination of static tendon and joint forces. Extrinsic and intrinsic tendons of the thumb were found to sustain forces of up to 10.0 and thirty kilograms during pinch, producing five kilograms of force at the thumb tip and forces of as much as fifty kilograms during grasp. The joint compression (contact) forces averaged three kilograms of force at the interphalangeal joint, 5.4 kilograms at the metacarpophalangeal joint, and 12.0 kilograms at the carpometacarpal joint during simple pinch (one kilogram of applied force). Compression forces of as much as 120 kilograms may occur at the carpometacarpal joint during strong grasp.