Muscle fibres that cross the proposed line of incision of the flexor retinaculum at carpal tunnel decompression can be a source of confusion, particularly for the less experienced surgeon. We investigated how frequently muscle fibres crossed the line of incision on the palmar surface of the flexor retinaculum at carpal tunnel decompression in 143 hands, and dissected 103 cadaver hands to study the origin and insertion of these muscle fibres. The line of incision was defined as a longitudinal line between thenar and hypothenar eminences along the third web space, with the wrist in neutral radioulnar deviation and the fingers in extension. Muscle fibres crossing the line of incision were absent in 50% of the operated hands, 2-10 mm wide in 39% and more than 10 mm wide in 11%. In the cadaver hands the proportions were similar at 50%, 35% and 15%, respectively. The fibres were extensions of the thenar and hypothenar muscles and did not appear to represent a separate anomalous muscle.