Measures of cerebral blood flow (CBF) are often used to examine cerebral physiology after sport-related concussion. Carbon dioxide modulates CBF and determines resting vascular tension yet studies rarely account for this. This study examined the effect of the end tidal partial pressure of carbon dioxide (PETCO2) on CBF in athletes. PETCO2 accounted for 14% of the variance in CBF and this increased to 37% when age and sex were included. No prior studies examining SRC and CBF have accounted for resting PETCO2. Future studies should move from univariate to multivariate methods to ensure that CBF-based estimates are interpreted correctly.