PRIMARY OBJECTIVE: Recent findings suggest breathing may negatively affect performance after concussion. The aim of the study was to examine group differences between individuals with and without post-concussion syndrome (PCS) in end-tidal carbon dioxide (ETCO2) and respiratory rate (RR) during neurocognitive and physical tasks. HYPOTHESIS: Individuals in the PCS group will have elevated ETCO2 and lower RR compared to the control group. METHODS: A quasi-experimental study was used in which a capnography unit collected ETCO2 and RR breathing data from 22 participants (control = 17; PCS = 5) during rest, the completion of the Immediate Post-Concussion Assessment and Cognitive Test (ImPACT) battery and while slow and fast walking. Four repeated measures mixed factorial ANOVAs, with an alpha level set at p </= 0.05, were used to examine group and task differences. RESULTS: Both groups displayed similar breathing responses to cognitive and physical stress; however, PCS participants had significantly elevated ETCO2 throughout the study (during rest, the ImPACT and while walking) compared to the control. Conversely, no significant group differences were found in RR. CONCLUSION: Cerebral physiological alterations and deficits in the autonomic nervous system may cause abnormal ETCO2 in individuals after concussion. Future research evaluating breathing patterns in PCS may be warranted.