While substantial scholarly attention has been paid to the beneficial consequences of transformational leadership and the conditions in which this leadership style is most effective, there is a remarkable shortage of research on the contextual antecedents of transformational leadership behavior itself. To address this gap, a laboratory experiment was conducted in which we tested the relationship between task complexity and the emergence of transformational leadership behavior. In this experiment, 111 participants were divided in groups of three (comprising one leader and two subordinates), and were instructed to solve three decision-making tasks with varying levels of task complexity. Results indicated that task complexity was negatively related to transformational leadership behavior, and that this relationship was partially mediated by the leader's state core self-evaluations. In other words, when leaders encounter tasks that are overwhelmingly complex, they act in less transformational ways because they momentarily lack the psychological resources to do so. Limitations and future research directions are discussed.