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To determine the nature of sensory change and its association with pain and allodynia in acute herpes zoster.Prospective clinical study.One hundred thirteen immunocompetent patients with acute herpes zoster.Onset, intensity, and quality of pain and severity of rash were recorded. Quantitative somatosensory testing for tactile and thermal thresholds, qualitative pinprick testing, and testing of dynamic and static allodynia were performed within the affected dermatome, its mirror-image dermatome, and in an adjacent dermatome bilaterally.Acute pain was reported as severe in 50%, moderate in 29%, mild in 12%, and absent in 9% of patients. Preherpetic pain (median = 4 days, range = 1-60 days) was experienced by 71%. Mechanical allodynia, dynamic, static, or both, was found in 37% of patients and was noted to extend one or more dermatomes outside the rash in 12%. In the affected dermatomes, thresholds were elevated for warmth and cold, lowered for heat pain, and unchanged for touch when compared with the contralateral side. Logistic regression analyses showed that compression-evoked allodynia, brush-evoked allodynia, and the history of preherpetic pain were more frequently encountered in patients with severe pain. Sensory threshold changes were not associated with the severity of pain or rash or with the presence of allodynia.Pain, allodynia, and altered sensation are common features of acute herpes zoster. They are likely to result primarily from widespread neural inflammation within the affected afferent system. The sensory changes found in acute herpes zoster are different from those reported in published studies on postherpetic neuralgia and suggest sensitization phenomena and preservation of tactile functions rather than major neural damage. The exact mechanisms for acute herpes zoster pain, however, remain speculative.