Background: Little is known about the genetic determinants of sensitive parenting. Two earlier studies examined the effect of the serotonin transporter polymorphism (5-HTTLPR) on sensitive parenting, but reported opposite results. In a large cohort we further examined whether 5-HTTLPR is a predictor of observed maternal sensitivity and whether observed child social fearfulness moderates the effect of 5-HTTLPR on maternal sensitivity. Methods: The population-based cohort consisted of 767 mother-child dyads. Maternal sensitivity was repeatedly observed at the child's age of 14 months, 36 months and 48 months. Sensitivity was coded using the Ainsworth's rating scales for sensitivity and cooperation and the revised Erickson rating scales for Supportive presence and Intrusiveness. Child social fearfulness was observed using the Stranger Approach episode of the Laboratory Temperament Assessment Battery at 36 months. Results:: Repeated measurement analyses showed a consistent main effect of maternal 5-HTTLPR on sensitivity; mothers carrying the S-allele were more sensitive toward their children (p = 0.005). This effect was not explained by the child's 5-HTTLPR genotype. We found no evidence that child social fearfulness moderated the effect of 5-HTTLPR on sensitivity. Conclusions: This study suggests that variations in maternal 5-HTTLPR genotype appear to be involved in the etiology of parenting behavior. The observed effects of this genetic variation are consistent with the notion that parenting may have a genetic component, but large studies are needed to find the specific small molecular effects.