The extensor digitorum brevis manus (EDBM) is a supernumerary muscle in the dorsum of the hand frequently misdiagnosed as a dorsal wrist ganglion, exostosis, tendon sheath cyst or synovitis. Its presence in a living subject, confirmed by magnetic resonance imaging (MRI), is presented together with a review of the hitherto reported cases and the results of an anatomical study on 128 adult human cadavers (59 males and 69 females). The EDBM was found in three (2.3%) of the 128 cadavers. It occurred in two (3.4%) of the 59 male cadavers (one bilateral and one unilateral on the right side) and in one (1.5%) of the 69 female cadavers (unilateral on the left side). Consequently, the EDBM was found in four (1.6%) of the 256 upper limbs, It originated from the dorsal wrist capsule within the compartment deep to the extensor retinaculum for the extensor digitorum and inserted into the extensor hood of the index finger in one case and into that of the middle finger in three cases. In both hands of the living subject, the origin was similar but the insertion was into the index and middle fingers. In all cases, it was innervated by the posterior interosseous nerve and its blood supply was provided by the posterior interosseous artery. © 2002 Wiley-Liss, Inc.