An inverse relation between cortisol (re)activily 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 = 72 studies, N = 5,480) and one for cortisol reactivity to a stressor (k = 29 studies, N = 2,601). No association was found between cortisol reactivity and externalizing behaviors (r = -.04, 95% CI = -.11, .02). However, the relation between basal cortisol and externalizing behavior was significant but small (r = -.05, 95% CI = -.10, -.002). The age of the children significantly moderated this relation: Externalizing behavior was associated with higher basal cortisol (hyperactivity) in preschoolers (r =.09, 95% CI =.002, .17), and with lower basal cortisol (hypoactivity) in elementary school-aged children (r = -.14, 95% CI = -.19, -.08). There was no significant relation between cortisol and externalizing behavior in adolescents. © 2008 Wiley Periodicals, Inc.