According to Belsky's process model of parenting, parents' personality represents the most important factor influencing parenting and child development. While an extensive literature has empirically corroborated the role of irritability traits in predicting aggressive behaviors in laboratory-based studies, only a few studies have examined the role of irritability in predicting aggressive behaviors within family contexts. The present study addressed this gap by examining the longitudinal association between maternal irritability and harsh parenting. Referencing latent state-trait theory (LST), first we estimated the amount of variance in mothers' irritability due to trait and state components, and, next, we examined the relation between mothers' irritability (both at trait- and state- levels) and harsh parenting over time. A sample of 204 mothers from Naples and Rome provided data over 5 years in four waves. Mothers averaged 40.30 years (SD = 5.33) at Time 1 and 44.01 years (SD = 5.43) at Time 4. Their children (50% girls) were 9.45 years (SD = 0.74) at Time 1 and 13.18 years (SD = 0.66) at Time 4. Results of LST analysis showed that, on average, 39% of variability in irritability was due to trait-like factors and only 12% to state-like factors. A multitrait-multistate model revealed that the irritability trait associated with mother's lack of control predicted her harsh parenting 1-year later, while controlling for the rank-order stability of harsh parenting.