The goals of this study were to examine the relations between and trajectories of mothers' and children's social positive expressivity. Mothers' and children's positive expressivity (PE) were observed annually for 4 years beginning when children were approximately 18 months old (n = 247; 110 girls). Based on correlations, there was evidence of rank-order stability in children's and mothers' PE. Based on growth curve analyses, mothers' and children's PE followed curvilinear trajectories; thus, mean-level instability was found. Children's PE during a free-play interaction with their mothers increased then decreased slightly whereas mothers' affect during the same task decreased then stabilized. Children's PE during a joy-inducing situation (i.e., bubbles) with an experimenter slightly decreased and then increased. In panel models, there was no evidence of prediction over time across children's and mothers' PE when taking stability into account. These unique trajectories and relations provide insight into the developmental pattern of young children's and their mothers' PE elicited within social contexts.