Injury to the ulnar collateral ligament of the thumb, once a common chronic occupational injury occurring in British gamekeepers, is now most frequently an acute sports-related injury in skiers, football players and other athletes. Differentiation of complete versus partial tears of the ligament is crucial because the treatment for complete tears is surgical. Radiographic evaluation, while important, has a limited role. Partial ligamentous tears, or those associated with uncomplicated avulsion fractures of the proximal phalanx, can be adequately treated by the family physician using simple shortarm thumb spica casting.