Although many studies have examined hardiness as a protective factor against stress reactions, to the authors' knowledge no studies have yet examined the impact of stress reactions on hardiness. However, there is theoretical support for this notion. In this longitudinal study of 1,571 Marine recruits who participated in a highly stressful training program, the authors applied regression-based cross-lagged analyses to examine associations between stress reactions and hardiness over time for both men and women, and they investigated social support as a moderator of these relationships. Men who were hardier at Time 1 (T1) reported lower stress reactions at Time 2 (T2), and men who experienced more stress reactions at T1 were less hardy at T2. Although the negative impact of stress reactions on hardiness was strongest when social support was low for both genders, stress reactions predicted enhanced hardiness when social support was high for women only.