The authors examined the relations of maternal supportive parenting to effortful control and internalizing problems (i.e., separation distress, inhibition to novelty), externalizing problems, and social competence when toddlers were 18 months old (n = 256) and a year later (n = 230). Mothers completed the Coping With Toddlers' Negative Emotions Scale, and their sensitivity and warmth were observed. Toddlers' effortful control was measured with a delay task and adults' reports (Early Childhood Behavior Questionnaire). Toddlers' social functioning was assessed with the Infant/Toddler Social and Emotional Assessment. Within each age, children's regulation significantly mediated the relation between supportive parenting and low levels of externalizing problems and separation distress, and high social competence. When using stronger tests of mediation, controlling for stability over time, the authors found only partial evidence for mediation. The findings suggest these relations may be set at an early age.