In 246 hands in which the carpal tunnel was explored at operation, 29 variations in the course of the median nerve were found. Accessory branches at the distal portion of the tunnel were found in 18 hands, a high division of the median nerve in seven, and accessory branches proximal to the tunnel in four. Based upon these findings and including published reports of others, the variations were classified into four groups: I-variation in the course of the thenar branch; II-accessory branches at the distal portion of the carpal tunnel; III-high divisions of the median nerve; and IV-accessory branches proximal to the carpal canal. The thenar branch variations in 100 cadaver hands (Poisel) were extraligamentous in 46 percent, subligamentous in 31 percent, and transligamentous in 23 percent. The findings emphasize the importance of approaching the median nerve from the ulnar side when opening the carpal tunnel.