An inverse relation between cortisol (re)activity and externalizing behavior has been hypothesized, but research findings seem equivocal. We tested this hypo(re)activity hypothesis in two meta-analyses, one for basal cortisol (k ¼ 72 studies, N ¼ 5,480) and one for cortisol reactivity to a stressor (k ¼ 29 studies, N ¼ 2,601). No association was found between cortisol reactivity and externalizing behaviors (r ¼ À.04, 95% CI ¼ À.11, .02). However, the relation between basal cortisol and externalizing behavior was significant but small (r ¼ À.05, 95% CI ¼ À.10, À.002). The age of the children significantly moderated this relation: Externalizing behavior was associated with higher basal cortisol (hyperactivity) in preschoolers (r ¼.09, 95% CI ¼ .002, .17), and with lower basal cortisol (hypoactivity) in elementary school-aged children (r ¼ À.14, 95% CI ¼ À.19, À.08). There was no significant relation between cortisol and externalizing behavior in adolescents. ß