Herpes zoster infections are frequently complicated by a postherpetic neuropathy. Postherpetic neuralgia is one of the most troublesome disease in pain clinic. Current therapy includes tricyclic antidepressant, anticonvulsants, sympathetic and somatic nerve blocks, and transcutaneous electrical nerve stimulation(TENS). However, in a number of case, the illness dose not respond to treatment very well. The N-methyl-D-aspartate(NMDA) receptor is one of the receptor subtypes of the excitatory amino acids(EAA) glutamate, and seems to play a significant role in the pathogenesis of nerve injury pain and neuropathic pain. The non-competitive NMDA receptor blocker ketamine reduced continuous and evoked pain in patients with injury of the peripheral or central nervous system. We present three cases in which patients suffering from postherpetic neuralgia did not respond to conventional therapy and in whom continuous intravenous infusion of ketamine reduced severe pain.