Associations between parents' dispositional optimism-pessimism (LOT-R) and their ratings of their children's behaviour were studied prospectively from infancy (M = 6.3, SD = 1.3 months) to middle childhood (M = 5.5, SD = 0.23 years) (n = 212). One parent's higher optimism (overall LOT-R and component score) and/or lower pessimism (component score) at infancy predicted the same parent's own but not the other parent's ratings of the child's behaviour as less internalising and less externalising, and socially more competent and greater in self-mastery in middle childhood, even when controlling for child's positive and negative affectivity 5 years earlier. Ratings of lower negative affectivity in their infant predicted the same parent's increasing optimism and decreasing pessimism over 5 years. The associations between parental optimism and the child's social competence and self-mastery survived after adjustments for parental neuroticism and depressive symptoms. Neither parent nor child gender systematically moderated the associations. The current findings shed light on the developmental paths of children's positive behavioural outcomes. (n = 144). Copyright © 2006 John Wiley & Sons, Ltd.