Objectives It has been recently reported that boxing and kickboxing may cause pituitary dysfunction, GH deficiency in particular. The strong link between poor cognitive performance and GH deficiency due to causes other than head trauma and the improvement of cognitive function after GH replacement therapy have been previously shown. P300 auditory event-related potential (ERP) measure is widely used to evaluate cognitive performance. In this study, we investigated the relation between the GH-IGF-I axis and cognitive performance in boxers and kickboxers. Design and patients Forty-one actively competing or retired male boxers (n: 27) and kickboxers (n: 14) with a mean age of 29 center dot 04 +/- 9 center dot 30year and 14age- and education-matched healthy male controls were included in the study. For neuropsychological tests, the mini-mental state examination (MMSE) and Quality of Life Assessment of GH Deficiency in Adults (QoL-AGHDA) questionnaires were administered. Moreover, cognitive performance was evaluated according to P300 ERPs. Results Nine of 41 (21 center dot 9%) athletes had GH deficiency. P300 amplitudes were lower at all electrode sites in the GH-deficient group than in controls, and the differences were statistically significant at Fz and Oz electrode sites (P<0 center dot 05). When GH-deficient athletes were compared with GH-sufficient athletes, the P300 amplitudes were lower at all electrode sites in the GH-deficient group; these differences were statistically significant at Fz, Pz and Cz electrode sites (P<0 center dot 05). In all athletes, there were significant negative correlations between IGF-I levels vs P300 latencies, and there were significant positive correlations between IGF-I levels vs P300 amplitudes (P<0 center dot 05). Conclusion This study provides the first electrophysiological evidence for the close relation between the P300 ERPs and the GH-IGF-I axis in boxers and kickboxers.