Introduction Hyperventilation (HV) is a routinely performed activation procedure during electroencephalogram (EEG) recordings, and useful to provoke 3Hz spike wave complex diagnostic for absence seizure. During HV, various rhythmic slow activities can occur in normal children, which are explained by cerebral vasoconstriction at low carbon dioxide concentration in the blood. When there is change in consciousness during HV with corresponding EEG criteria, it is defined as HV induced high amplitude rhythmic slowing with altered awareness (HIHARS). EEG criteria for HIHARS are as follows: 1) high amplitude (＞100μV), 2) ~2.5 to 5 Hz generalized rhythmic slowing, and 3) duration of ≥3 seconds(s). Clinically, this condition is indistinguishable from absence seizure because of accompanied automatisms like yawing, smiling, fidgeting. This entity is known to be non-epileptic phenomenon and remits spontaneously in most patients. Therefore, clinicians need to know HIHARS as a clinical mimicker of absence seizure to avoid unnecessary treatment. Case A previously healthy 6-year-old girl visited clinic due to one episode of motionless staring accompanied by eyelid blinking and yawing during excessive crying. She had enteroviral meningitis 2 years ago. Several brief episodes of looking drowsy with eyelid blinking were observed during HV induction at the office. Clinical diagnosis of absence seizure was made. During the EEG recordings, there were 4 times of high amplitude (＞100μV), 2 to 4 Hz generalized rhythmic activity lasting for 18 to 25s during 5 minutes of HV. During that build-up on EEG, the patient stopped HV and showed altered awareness with yawing. Brain magnetic resonance imaging with angiography was performed to exclude any possible cerebrovascular disease such as Moyamoya disease, and which was unremarkable. Her routine laboratory test results were normal. She was diagnosed as HIHARS. No treatment was tried. During the 2 weeks of follow-up, she was remained symptom free without HV. Conclusion HIHARS is a condition resembling absence seizure clinically. Only EEG can make a right diagnosis for the patients presenting altered awareness with automatism during HV.