Objective: To investigate the relationship between preschool depression severity, observed behavior, and parental emotional support in a population of 3.0- to 5.6-year-olds and their mothers. Method: One hundred fifty preschoolers who underwent a comprehensive mental health assessment during which DSM-IV diagnoses were derived were included in this analysis. Child and parent behaviors during challenging structured dyadic tasks were systematically coded. Dyads with preschoolers in three diagnostic groups of interest were explored: depression, disruptive, and healthy. Depression severity sum scores were derived for children in all of the groups. Results: Depression severity accounted for a significant (p less than 0.05) portion of the variance in preschoolers' persistence, compliance, and enthusiasm during dyadic tasks after controlling for the effects of age and gender. Depression severity was also significantly associated with parental emotional support, which was itself associated with all three preschool behaviors. When the effect of parental support was controlled for statistically, however, preschoolers' depression severity was no longer significantly associated with observed persistence or compliance, whereas the relationship between depression severity and enthusiasm remained significant. Conclusions: Maternal emotional support mediated the relationship between preschoolers' depression severity and their persistence and compliance but not the relationship between depression severity and enthusiasm. Findings have important clinical implications because they suggest that both external relational and internal child factors may be operating in preschool depression. (Contains 4 tables.)