A study of the innervation of the brachialis muscle was carried out on 45 male and 31 female Thai cadavers between the ages of 15 and 92 years (mean = 59 years). The dissections revealed that all brachialis muscles received innervation from the musculocutaneous nerve and that 81.6% were also innervated by a branch from the radial nerve. Among the brachialis muscles with a dual nerve supply, two patterns of branching from the radial nerve were observed: in one pattern the branch to the brachialis had a descending course (58%) and in the other pattern the nerve ascended or recurred (42%) to innervate the muscle. The radial nerve branch penetrated the inferolateral part of the brachialis muscle in 83% of cases (103/124) and its middle third in 17% of specimens (21/124). The basis for the dual innervation may result from fusion of two different embryonic muscular primordia: the ventral (flexor) and the dorsal (extensor) muscle masses. In contrast to a brachialis muscle innervated by the musculocutaneous nerve only, a muscle with dual innervation may be spared significant denervation by an anterior approach to the humerus through a longitudinally bisected muscle. In a dually innervated muscle, however, separation of the brachialis and brachioradialis muscles during surgery may put the radial nerve branch to the brachialis at risk.