We measured ERPs of professional boxers in a Go/No-Go task, comparing them to fencers and non-athletes. Results showed that fencing improved attention and motor response control, but boxing did not. More strikingly, in boxers, as in brain trauma patients, the P3 component was delayed and reduced. The P3 delay of boxers was correlated with the amount of performed sport exercise. Furthermore, in terms of behavior, boxers showed increased intra-individual variability and switch cost. Results were consistent with the hypothesis of specific impairment at the level of response inhibition processing. We suggest that this impairment is derived from the cumulative effect of blows to the head. The changes found in boxers suggest that ERPs and reaction times may be a tool for early detection of specific brain dysfunction.