Rationale: Stress affects flow-experience, but the mediating psychobiological mechanisms remain unknown. Previous studies showed an association between flow-experience and endogenous cortisol levels, suggesting an inverted, u-shaped relation between flow-experience and cortisol. However, these studies could not exclude effects of other stress factors. Objectives: The aim of this experiment was, therefore, to test the isolated effect of cortisol on flow-experience, independent of concomitant physiological and psychological stress responses, via controlled administration of exogenous cortisol. Methods: Sixty-four young healthy subjects (32 males, 32 females) participated in the experiment. According to a double-blind, randomized, placebo-controlled, cross-over design, they received 20 mg oral cortisol on 1 day and placebo on the other day, respectively, with a time distance of 1 week between the experimental days. One hour after cortisol administration, participants engaged in the computer game Pacman. Pacman was delivered in five blocks of randomly differing difficulty levels. One block lasted 5 min. At the end of each block, participants rated flow-experience by the Flow Short Scale. Data was analyzed with hierarchical linear modeling. Subjects were not able to predict whether the pill they received contained cortisol or placebo. Results: Overall, results revealed a negative effect of oral 20 mg cortisol on flow-experience, with no differences between males and females. Conclusions: This study is the first to show that exogenous cortisol in a dose corresponding to a severe stressor impairs flow-experience. The observed negative effect of high cortisol dosage on experienced flow underlines recent findings of an inverted u-shaped relationship between cortisol and flow.