This study contributes to literature on leadership by linking the full-range leadership behaviors (transformational, transactional, and laissez-faire leadership) with an objective indicator of employees' stress, namely cortisol, assessed via hair. Hair cortisol is a biological indicator of stress, providing an innovative means of displaying the cortisol concentration of the human body over time. Building on a role ambiguity framework, this study explores the double-edged relationship between full-range leader behaviors and followers' stress by focusing on clarifying and ambiguity-increasing aspects of these leader behaviors. One-hundred-twenty-nine participants provided information on their leaders' full-range leadership behaviors together with a hair sample. Results show leader behaviors have significant relationships with followers' hair cortisol level. Results confirm our hypotheses and reveal two different patterns of leader behaviors with regard to stress: a stress-reducing as well as a stress-promoting pattern of leader behaviors. Results are discussed in the context of leadership research and stress theory, and limitations together with implications for future research are presented.