This study examined the interplay between interparental conflict and child cortisol reactivity to interparental conflict in predicting child maladjustment in a sample of 178 families and their kindergarten children. Consistent with the allostatic load hypothesis (McEwen & Stellar, 1993), results indicated that interparental conflict was indirectly related to child maladjustment through its association with individual differences in child cortisol reactivity. Analyses indicated that the multimethod assessment of interparental conflict was associated with lower levels of child cortisol reactivity to a simulated phone conflict between parents. Diminished cortisol reactivity, in turn, predicted increases in parental reports of child externalizing symptoms over a 2-year period. Associations between interparental conflict, child cortisol reactivity, and child externalizing symptoms remained robust even after demographic factors and other family processes were taken into account.