Iatrogenic laceration of the radial digital nerve of the thumb during trigger release occurred in four patients. Histologic cross sections through the thumb in cadaveric specimens at the level of the metacarpophalangeal crease showed the radial digital nerve to be 2.19 mm beneath the dermis and 1.15 mm directly anterior to the radial sesamoid bone. In this position the nerve is vulnerable to transection when it is trapped between the sesamoid and the knife blade.