While parenting behaviours have direct effects on children's behavioural outcomes, other, more distal factors also may be shaping the way a mother handles parenting responsibilities. Dispositional factors are likely to be a major influence in determining how one parents. Although researchers have studied the relationships among maternal dispositional factors, parenting, and child behaviours, few studies have examined these relationships when the child is at developmental risk. Children with developmental delays evidence elevated clinical level behaviour problems, so this group is of primary interest in the search for precursors to psychopathology. The present study examined how the maternal dispositional trait of self-mastery, as well as supportive and non-supportive parenting, relate to behaviour problems in young children with and without developmental delay. Participants were 225 families, drawn from Central Pennsylvania and Southern California. The children, all aged 4 years, were classified as delayed (n = 97) or non-delayed (n = 128). The Self-Mastery Scale measured perceived level of control over life events. The Coping with Children's Negative Emotions Scale measured different ways parents perceive themselves as reacting to their children's distress and negative affect. The Child Behavior Checklist assessed children's behaviour problems. Delayed condition mothers reported significantly more child behaviour problems than non-delayed condition mothers; the two conditions did not differ in self-mastery, supportive parenting, or non-supportive parenting. Self-mastery, non- supportive parenting reactions, and child behaviour problems all related significantly to one another. For the sample as a whole and within the delayed condition, the association between self-mastery and child behaviour problems was partially mediated by non-supportive parenting reactions, although self-mastery was still significantly associated with problem behaviour. In the non-delayed condition, although significant relationships also were found among the variables of interest, non-supportive parenting did not have a significant main or mediation effect. Delay status moderated the relationship between negative parenting reactions and child behaviour problems, assessed by the Child Behavior Checklist Total and Internalizing scores. When mothers displayed low levels of non-supportive reactions, children in the delayed and non-delayed groups had similar levels of total problem behaviour. However, when mothers were medium or high in non-supportive reactions, children in the delayed group had much higher levels of problem behaviours than those in the non-delayed group. The present study extended research on parental dispositional factors and parenting by measuring self-mastery as a global personality trait rather than measuring self-efficacy related specifically to childrearing. Moreover, relationships were examined for both developmentally delayed and non-delayed samples, allowing for a clearer understanding of the influences on problem behaviours in children with developmental delays. The findings support the view that parenting behaviours have a greater impact on children at developmental risk.