BACKGROUND: Being a subjective symptom, an objective evaluation of pain and severity is important in the diagnosis and detection of treatment outcome. This study examined the usefulness of infrared thermography for the objective evaluation of pain, irrespective of the original disease. METHODS: Patients with unilateral pain who underwent nerve block were randomly selected. Infrared thermography was performed and subjective pain site and severity were assessed before and after nerve block. RESULTS: The temperature difference between the pain site and the contralateral site was significantly correlated with subjective pain severity before and after block (P < 0.01). Improvements in VAS were correlated with temperature difference decrement between both sides after nerve block (P <0.05). CONCLUSIONS: Infrared thermal imaging can demonstrate subjective pain objectively. Thermal differences between the pain sites and the contralateral sites are an indicator of pain scale in a patient with ipsilateral pain. Moreover the thermal difference may be a useful means of determining outcome.