Bilaterality in acute low-tone sensorineural hearing loss (ALHL) is more generally recognized than that in idiopathic sudden sensorineural hearing loss. Subjects were 274 patients diagnosed with ALHL based on criteria of a study group of the Ministry of Health, Labor and Welfare of Japan, i.e., total of 3 low tone hearing of 70dB or more and, a total of 3 high-tone hearing of 60dB or less, and treated at the departments of otolaryngology at Yamanashi University and Suwa Central Hospital. ALHL involving bilateral ears symptoms and/or bilateral hearing impairment conforming to diagnostic criteria was selected and summarized. Clinical ear symptoms, clinical test results, and hearing levels (total 3 low tone hearing, 1kHz, and total of 3 high-tone hearing) were statistically analyzed. We also reviewed Japanese clinical reports of ALHL that include bilateral cases. In 32 cases (11.7%) of 274 cases, both ear symptoms and hearing impairment were bilateral. In 22 (8.0%) of the 274, bilateral ear symptoms were present, but showed unilateral hearing loss conforming to diagnostic criteria. Another 22 (8.0%) out the 274 reported unilateral ear symptoms, but hearing tests indicated bilateral ALHL. A total of 76 cases (27.7%) of the 274 had bilaterality in either ear symptoms or hearing loss. Our review indicated that 9.0% (162 of 1803) ALHL patients were bilaterally affected, possibly indicating that AIHL includes a larger number of bilateral cases than currently assumed, if the opposite side were given a especially detailed clinical interview. Statistical analysis (Mann Whitney test, P<0.01) of hearing of unilateral cases indicated that 3 low tone hearing was more affected than 3 high-tone hearing, even on the normal side. These results indicate that ALHL tends to be bilateral, possibly due to the mechanism of pathogenesis, and also that the mechanism may include both local and general conditions. This more closely resembles Meniere's disease than idiopathic sudden sensorineural hearing loss. Both sides of bilateral cases were not usually the same in hearing patterns, glycerol test results, or prognosis. A statistically significant difference (Mann Whitney test, P<0.01) in total of 3 low tone hearing was seen between worse and better sides in bilateral cases. The degree of disease on both sides in bilateral cases thus was not always the same. Bilateral cases may result from both the influence of general conditions such as fatigue, stress, and lack of sleep and local conditions such as pathogenesis of endolymphatic hydrops that may cause differences in both ears. No clear difference was seen in clinical symptoms, hearing levels, and clinical examination, e.g., Schellong and glyceol tests, between unilateral and bilateral cases. Bilateral cases had a poorer prognosis (lower complete recovery ratio; chi2 test P<0.01) than unilateral cases. Our results indicated that cautious evaluation of opposite ear is necessary in diagnosis, treatment, and prognostication of ALHL.