This study aimed to investigate the effect of hypocapnia on attentional performance. Hyperventilation, producing hypocapnia, is associated with physiological changes in the brain and with subjective symptoms of dizziness, concentration problems and derealization. In this study (N=42), we examined cognitive performance on a Stroop-like task, following either 3 min of hypocapnic or normocapnic overbreathing. Both overbreathing trials were run on separate days, each preceded by a baseline trial with the same task during normal breathing. More and other symptoms were reported after hypocapnia compared to normocapnia. Also, more errors were made and progressively slower reaction times (RT's) were observed during recovery from hypocapnia. These performance deficits were only found in participants characterized by apneas. The number of symptoms did not correlate with RT's or errors. The pattern of data suggested that hypoxia, as a result of apneas during recovery from hypocapnia, caused the cognitive performance deficit.