Intracarpal canal interstitial fluid pressures were determined in the wrists of 22 patients with 23 Colles' fractures. Mean values were 18 mm Hg in neutral wrist position, 27 mm Hg in 20° flexion, 47 mm Hg in 40° flexion, and 35 mm Hg in 20° extension. Ten of 23 wrists (45%) had pressures greater than 40 mm Hg in 40° of flexion. Because of the high incidence of median neuropathy associated with immobilization of wrist fractures in a position of marked flexion, an awareness of the magnitude of increased interstitial fluid pressure should lead to alternative methods of treatment in many cases.