Objective: To evaluate the clinical conditions causing an elevation in the click-evoked summation potential (SP)/action potential (AP) amplitude ratio (SP/AP ratio), the cause of the SP enhancement in Ménière’s disease (MD) and the diagnostic efficacy of electrocochleography (ECoG) were discussed. Study Design: Retrospective case review. Setting: An outpatient clinic of the Otolaryngology Department of Kochi Medical School. Patients: ECoG testing was performed in 632 patients (727 ears) with vertigo/dizziness and/or deafness over a 10-year period (1995–2005). Among them, 334 patients had diagnoses of definite MD, including 95 cases of bilateral involvement. Main Outcome Measures: Audiological thresholds and SP/AP ratio. Results: An enhanced SP was observed in 56.3% of patients with MD. The incidence of an enhanced SP was low in patients for whom the disease duration was 2 years or less and the frequency of attacks was once a year or less, but was significantly higher in cases where the disease duration was 2 years or longer and/or the frequency of attacks was several times a year (Games-Howell test, p < 0.05). The incidence of an enhanced SP was significantly elevated in cases with pure-tone average exceeding 31 dB (Kendall’s rank correlation test, p < 0.001). However, the enhanced SP, once it appeared, did not always disappear in spite of hearing improvement. Hearing improvement induced by the glycerol test also produced no alteration in an SP/AP ratio, and there was no significant difference between the glycerol test results and the incidence of an enhanced SP (χ2 goodness-of-fit test). Conclusions: The longer the patients were symptomatic or the severer the ear symptoms, the more likely the ECoG was to be positive. The abnormally elevated SP, once it had appeared, persisted for long periods. Spontaneous and glycerol-induced hearing gains did not result in a decrease in SP/AP ratio. These clinical characteristics of ECoG seem to indicate that the enhanced SP in MD might be caused by the malfunction of hair cells, not by the displacement of the basilar membrane toward the scala tympani.