The distal ulnar tunnel is a region of the wrist 4-4.5 cm in length in which the ulnar nerve is particularly vulnerable to external compression. The relation of the internal topography of the nerve to the structures comprising the tunnel provides a basis for dividing the tunnel into three zones. Zone 1 is that portion of the tunnel proximal to the bifurcation of the ulnar nerve. Zone 2 encompasses the deep motor branch of the nerve, and Zone 3 surrounds the superficial branch. A review of the literature of ulnar nerve compression lesions confirmed expectations based on the regional anatomy. Zone 1 lesions included all (39) cases of combined motor and sensory deficits, one case of pure motor paralysis, and seven cases of sensory deficits. All Zone 2 lesions (36 cases) resulted in paralysis of the intrinsic muscles. Whether or not the hypothenar muscles were affected was dependent upon the location of the lesions within Zone 2. Zone 3 lesions caused sensory deficits only. Combined motor and sensory loss was most often caused by compression from deep to the nerve, while pure sensory deficits were a result of compression lesions lying superficial to the nerve.