This study describes a method of detecting first metatarsal pronation on the basis of the movement of the inferior tuberosity of the base of 20 cadaveric first metatarsals at 0 degrees, 10 degrees, 20 degrees and 30 degrees pronation. On pronation, the inferior tuberosity of the base of the first metatarsal moved lateral to the mid-line axis. At 10 degrees, the tuberosity pointed to the junction of the inner third and outer two-thirds of a line between the midpoint and lateral tubercle of the base. At 20 degrees, it pointed to the junction of the inner two-thirds and outer third of that line. At 30 degrees, it pointed to the outer margin of the lateral third. Using these features, the amount of first metatarsal pronation in 100 consecutive weight-bearing views of feet was recorded and plotted against the corresponding intermetatarsal angles in those feet. Four of 43 patients with an intermetatarsal angle of less than 9 degrees had pronation greater than 10 degrees, 48 of 57 patients with an intermetatarsal angle greater than 9 degrees had pronation greater than 10 degrees (P < 0.001). As intermetatarsal angles increase, the amount of first metatarsal pronation increases (r = 0.69). Pronation and varus deviation of the first metatarsal are linked; both alter the tendon balance maintaining proximal phalanx alignment and lead to the development of hallux valgus.