Abstract Event-Related Potential (ERP) due to vocalization of a single syllable was recorded in normal adults and in persons with Down syndrome, and their cerebral processes during single syllable vocalization were studied. EEGs were recorded from 20 locations on the scalp including Broca's and Wernicke's areas. EMG of orbicularis oris muscle and subject's voice through a microphone were also monitored during the EEG measurement, and were averaged after rectification. From EEGs averaged before and after vocalizing a Japanese syllable, /u/ or /mu/, we recognized Vocalization-Related Cortical Potential (VRCP) with Bereitschaftpotential (BP) before vocalization contains. Significant difference of BP due to vocalization of a vowel syllable (/u/) was not observed between subject groups. For vocalizing a Consonant–Vowel (CV) syllable (/mu/), however, BP amplitude in Down syndromes was significantly larger than normal adults. Therefore, the increased BP in Down syndromes might be brought by their enhanced effort and expectation to vocalize CV syllable. VRCP after the onset of vocalization may contain not only motor component related to control of vocalization organ but also auditory response to one's own voice as an auditory feedback. As people with Down syndrome vocalize louder voice with longer duration, motor and auditory components might be increased respectively and remain longer. Therefore, enhanced VRCP after the onset of vocalization in Down syndrome might be associated with inflamed cortical activation.