Background: Although low positive affect (PA) and high negative affect (NA) have been posited to predispose to depressive disorders, little is known about the developmental trajectories of these affects in children at familial risk for mood disorders. Methods: We examined 202 offspring of mothers who had a history of juvenile-onset unipolar depressive disorder (n = 60) or no history of major psychopathology (n = 80). Offspring participated in up to seven annual, structured laboratory tasks that were designed to elicit PA and NA. Results: Growth curve analyses revealed that PA increased linearly and similarly for all children from late infancy through age 9. However, there also were individual differences in early PA. Relative to control peers, offspring of mothers with lifetime unipolar depression had consistently lower levels of PA, and this association remained significant even when controlling for current maternal depression and maternal affect displays. Growth curve analyses also revealed a significant linear decrease in NA in children across time; however, there was no significant inter-individual variation either in early NA or rate of change in NA. Conclusion: Attenuated PA (rather than excessive NA) may be an early vulnerability factor for eventual unipolar depressive disorder in at-risk children and may represent one pathway through which depression is transmitted.