The distal non-traumatic ulnar nerve compression syndrome can be dividded into two types: -i) the proximal (paralytic) type caused by compression in the "Loge de Guyon", with involvement of both the superficial sensory and the deep branch, including the motor branch to the hypothenar muscles, and, ii) the distal, more common, purely motor type (the deep ulnar branch syndrome) with paresis of the interossei and the adductor pollicis, and less commonly of the hypothenar muscles. Whereas the proximal type has various causes, particularly pressure and occupational trauma, the distal type is almost exclusively the result of extra-neural ganglion cysts. An accurate diagnosis is made possible by electromyography and nerve conduction studies. The sensory nerve action potentials, distal motor latency to the hypothenar and adductor pollicis muscles, and the corresponding muscle action potentials after nerve stimulation are pathologically altered, according to the type of lesion. Although spontaneous recovery may occur, operation is the treatment of choice, provided that simple pressure palsy is eliminated.