We examined transgenerational effects of maternal childhood adversity on child temperament and a functional promoter polymorphism, 5-HTTLPR, in the serotonin-transporter gene (SLC6A4) as potential moderators of such maternal influences in 154 mother-child dyads, recruited into a longitudinal birth cohort study. We examined the interactive effects of maternal childhood experience using an integrated measure derived from Childhood Trauma Questionnaire (CTQ) and Parental Bonding Index (PBI). Triallelic genotyping of 5-HTTLPR was performed. A measure of ‘negative emotionality/behavioural dysregulation’ was derived from the Early Childhood Behaviour Questionnaire at 18 and 36 months. Negative emotionality/behavioural dysregulation was highly stable between 18 and 36 months and predicted psychosocial problems at 60 months. After controlling multiple demographics as well as both previous and concurrent maternal depression there was a significant interaction effect of maternal childhood adversity and offspring 5-HTTLPR genotype on child negative emotionality/behavioural dysregulation (beta = 1.03, t11,115 = 2.71, P < .01). The results suggest a transgenerational effect of maternal developmental history on emotional function in the offspring, describing a pathway that likely contributes to the familial transmission of vulnerability for psychopathology.