Although reconstruction of the torn thumb metacarpophalangeal (MCP) joint ulnar collateral ligament (UCL) has been shown to reduce symptoms, final joint motion may be different from that of the uninjured state. It was hypothesized that nonanatomic repositioning of the UCL might affect joint motion; therefore, the effect of UCL attachment site on MCP range of motion was investigated. The UCL and MCP joint capsule were visualized in each of 8 fresh cadaveric hands without otherwise disrupting the joint. The centers of the ligament attachments were marked with pins and each specimen was mounted on a testing frame capable of applying loads through the flexor and extensor tendons. After measuring the flexion, extension, and radial and ulnar deviation ranges of motion of the intact specimen, the origin of the ligament (on the metacarpal) and an attached bone block were elevated and repositioned 2 mm proximal and 2 mm palmar and range of motion was tested. The origin was reattached in its anatomic location and the insertion of the ligament was similarly elevated and displaced 2 mm in distal, dorsal, and palmar directions. Compared with the intact joint, palmar placement of the UCL origin on the metacarpal increased radial deviation (from a mean of 18 degrees to a mean of 27 degrees); proximal placement of the origin decreased it (from 18 degrees to 10 degrees). Similarly, dorsal placement of the UCL insertion on the phalanx increased radial deviation (from 18 degrees to 25 degrees) and distal positioning of the insertion decreased it (from 18 degrees to 11 degrees). Relative to intact joint flexion range of motion (mean, 57 degrees), distal placement of the UCL phalangeal insertion restricted motion (mean, 47 degrees), as did palmar placement (mean, 48 degrees). Extension and ulnar deviation motions were unaffected by ligament attachment position. This study demonstrates that nonanatomic reconstruction of the UCL alters normal MCP joint range of motion.