The concentration of the protein S100B in serum is used as a brain damage marker in various conditions. We wanted to investigate whether a voluntary, prolonged apnea in trained breath-hold divers resulted in an increase of S100B in serum. Nine trained breath-hold divers performed a protocol mimicking the procedures they use during breath-hold training and competition, including extensive pre-apneic hyperventilation and glossopharyngeal insufflation, in order to perform a maximum-duration apnea, i.e. "static apnea" (average: 335 s, range: 281-403 s). Arterial blood samples were collected and cardiovascular variables recorded. The arterial partial pressures of O₂ and CO₂ (Pa_O2 and Pa_CO2) were 128 Torr and 20 Torr, respectively, at the start of apnea. The degree of asphyxia at the end of apnea was considerable, with Pa_O2 and Pa_CO2 reaching 28 Torr and 45 Torr, respectively. The concentration of S100B in serum transiently increased from 0.066 µg_*L^-1 at the start of apnea to 0.083 µg_*L^-1 after the apnea (P < 0.05). The increase in S100B is attributed to the asphyxia or to other physiological responses to apnea, for example increased blood pressure, and probably indicates a temporary opening of the blood-brain barrier. It is not possible to conclude that the observed increase in S100B levels in serum after a maximal-duration apnea reflects a serious injury to the brain, although the results raise concerns considering negative long-term effects. At least, the results indicate that prolonged, voluntary apnea affects the integrity of the central nervous system, and do not preclude cumulative effects.