Vestibular function was examined in 55 adult patients who underwent cochlear implant surgery. The percentages of the patients with normal results on an optokinetic nystagmus test and eye-tracking test before surgery were 85% and 82%, respectively. A caloric stimulation test was performed before surgery, and 67% of patients showed hypofunction or afunction. This same test was performed approximately 1 month after surgery, and 38% of patients showed functional deterioration. After surgery 47% of the patients reported some kind of subjective dizziness. The characteristics of dizziness were divided into three types: early, prolonged, and delayed. Fifty-eight percent of the patients had early type, 34% had prolonged type, and 8% had the delayed type. Eighteen percent of the patients felt dizziness when they used the implant device, indicating that electrical current spread from the implant device to the vestibular nerve. The other 82% of the patients did not report any relationship between the use of their cochlear implants and dizziness.