Twelve otherwise healthy patients with longstanding postherpetic neuralgia (PHN) were prospectively studied using clinical examination, infrared thermography and response to local anesthetic skin infiltration. All had at least 2 of 3 possible components to their PHN pain: continuous, neuralgic, or allodynic. In patients with allodynia, maximal reported pain and the location of maximal allodynia on sensory examination were largely overlapping and were often warm thermographically. Areas of dense sensory loss and skin scarring without allodynia were usually cool thermographically. Local anesthetic skin infiltration produced substantial pain relief in all 9 patients (essentially complete relief in 7) with allodynia: the 3 patients with predominantly continuous pain were not relieved. In 7 of 8 skin infiltration responders, the same dose of lidocaine i.m. in the deltoid muscle also produced significant, though less complete pain relief. These results suggest that PHN patients can be divided into at least 2 clinical groups: those with predominantly continuous pain localized to a region of significant sensory loss and those in whom allodynia is the most prominent sensory disturbance. The latter group has pain localized to areas with relatively preserved sensation. The differences in clinical features and response to lidocaine suggest that there are at least 2 different mechanisms contributing to the pain of PHN.