Objective: Mothers who attribute child misbehaviors to children’s intentions, and not to situational causes, show more hostile parenting behaviors. Why are some mothers more likely than others to make more hostile attributions (i.e., high intentional attributions and low situational attributions) when confronted with child challenging behaviors? We examined the relation between mothers’ perception of child challenging behaviors and their hostile attributions of child misbehaviors, with an emphasis on how maternal negative affect and resting vagal activity moderated this relation. Design: One hundred sixty mothers of 3- to 7-year-old children reported their perceptions of child problem behaviors, their attributions regarding child misbehaviors, and their temperamental negative affect. Mothers’ respiratory sinus arrhythmia was measured during resting state. Results: Maternal perceptions of child challenging behaviors were positively related to hostile maternal attributions, and this relation was strongest in mothers with high negative affect and low resting RSA. Conclusions: These findings indicate the importance of considering mothers’ affective and physiological attributes when examining social-cognitive processes in parenting.