Abstract: A system and a method are described for determining a stress level of a person based on the performance of the person in a game or puzzle. First, in a calibration phase the person is asked to judge his/her currently experienced stress level (120) and the person is asked to play (130) a game or puzzle resulting in a certain performance score. Subsequently in this calibration phase, the system links (150) the indicated stress level and the performance score. By repeating these steps a number of times, the system builds a table linking game performance scores and stress levels. The calibration phase will be executed initially and once every while to keep the system calibrated. Subsequently, in an operational phase, the person is occasionally asked (220) to play a game. Then, based on the current performance score and on the table assembled in the calibration phase, the system determines (250) the current stress level.