External and internal measurements were taken of the metacarpal bones of 25 fresh adult, Caucasian cadaver hands. The shape of the intramedullary canal of each of the five metacarpals was different. There was also great variation between metacarpals in both cortical thickness and the diameter of the canals. The volar cortical wall of all metacarpals was approximately 20% thicker than the dorsal cortex. With the exception of the thumb, there was no difference between the radial and ulnar cortical thickness. The data obtained from these studies suggested strongly that the shape and dimensions of the intramedullary canal should be taken into consideration when designing intramedullary fixation devices or prostheses.