Acute low-tone sensorineural hearing loss (ALHL) has the following three criteria; obscure origin, acute onset and sensorineural hearing loss limited to low frequencies. Sixteen cases of ALHL which were considered as cochlear hydrops using glycerol test and electrocochleogram were studied. All patients visited our department within two weeks after onset and were followed up for one year or more after initial examination. The subjective symptoms, the character of vestibular and hearing impairment and prognosis of 16 cases with ALHL were also investigated. The results were as follows. 1. Patients complained of ear fullness rather than hearing impairment. Four patients were unaware of hearing loss. 2. Recruitment phenomenon was found in all of 15 cases examined. Vestibular findings were mostly normal, except that spontaneous nystagmus was found in two cases and head-shaking nystagmus in one. 3. Recurrence and fluctuation of hearing impairment occurred in 14 cases. Three cases had an attack of rotatory vertigo once and two has diagnosed as Meniere's disease later. 4. During three months prior to last examination, hearing was stabilized in nine cases and continued to fluctuate in seven cases. In the former, hearing was improved in four cases, unchanged in three, and worsened in two. 5. Two patients underwent an endolymphatic sac operation and have had a good prognosis. 6. The authors suggest that most of ALHL should be considered as transient cochlear hydrops because the subjective symptom and audiological and vestibular findings of our cases are similar to those of cases which were reported as ALHL by other investigators. According to the findings of glycerol test and electrocochleogram, endolymphatic hydrops in ALHL is considered to exist in all turns of cochlea.