Although it is controversial that seizure duration can influence the efficacy of electroconvulsive therapy (ECT), a missed or brief seizure is considered less effective ECT. Of the background in the practice of ECT, hyperventilation may augment the seizure duration. To elucidate these hypotheses, we performed double-blind randomized controlled trial for 19 patients. They were divided into 2 groups, according to the end-tidal pressure of carbon dioxide (ETCO2): The moderate hyperventilation group with ETCO2 of 30 mm Hg and the normal ventilation group with ETCO2 of 40 mm Hg. ECT was performed under general anesthesia with propofol and suxamethonium. During ECT electroencephalogram (EEG) and electromyogram were recorded. The Global Assessment of Functioning scores were also analyzed before and after 6 sequential ECT. The moderate hyperventilation group showed a significant increase in EEG seizure duration in the first treatment compared with the normal ventilation group (P < 0.05). However, EEG seizure duration in the subsequent treatments and electromyogram seizure duration in all the treatments did not differ between 2 groups. The moderate hyperventilation did not prevent the increase in seizure threshold or shortening of seizure duration. No complications or sever adverse effects were observed after ECT in any of the 6 treatments. The Global Assessment of Functioning scores were not significantly changed with moderate hyperventilation. We conclude that moderate hyperventilation is safe and may be useful for seizure augmentation before the restimulation with higher intensities.