BACKGROUND: Bell's palsy is the most common disease of cranial nerve. While most electrodiagnostic tests can detect the abnormality of facial nerve several days later in Bell's palsy, blink reflex usually reveals the abnormality earlier than other tests. Therefore, we investigated the diagnostic usefulness of blink reflex in the early stage of Bell's palsy. METHODS: We performed a prospective investigation in patients with facial palsy. We enrolled patients with Bell's palsy who were evaluated within 7 days of symptom onset and excluded patients with secondary causes of facial palsy. We analyzed the findings of blink reflex according to age, sex, evaluation time, and severity of facial palsy. RESULTS: Of 320 consecutive patients with facial palsy, a total of 204 patients were enrolled. Blink reflex was normal for 10 patients and abnormal for 194 patients. The time interval between the symptom onset and the evaluation time was not associated with the result of blink reflex, but House-Brackmann grade was significantly related to the result of blink reflex (P<0.001). Patients with mild Bell's palsy often showed normal blink reflex. CONCLUSIONS: Our study shows that blink reflex is useful diagnostic test regardless of evaluation timing in the early stage of Bell's palsy, although it could be normal in patients with mild Bell's palsy.