Previous work by our group has shown that infant emotional vitality (EV), the lively expression of shared emotion both positive and negative, predicts cognitive and language abilities in toddlerhood. Specifically, infants who demonstrated a pattern of high emotional expression combined with high bids to their caregivers, fared significantly better on the Bayley II and Preschool Language Scales (PLS-3) at 2 years of age than infants who showed both low expression and low bids to mother. That study was conducted on a large, low-income, ethnically diverse sample. The current study was undertaken with a smaller but demographically similar sample as an effort to demonstrate the value of EV as a construct and to provide additional information about its links with later developmental outcomes. Replication that included a variation in the age at which EV was assessed provides support for the generalizability of the construct. In addition, this study examined EV's effects further into childhood than did the original study in order to insure they are not limited to a brief window in toddlerhood. The results indicate that over and above maternal psychological resources, EV expressed during positive/joyful and frustrating circumstances in 8-month-olds accounted for significant portions of variance in expressive language at 30 months and cognitive-developmental assessments at 24 and 36 months. This study supports EV as an important relational-emotional behaviour that increases experiences that optimize developmental outcomes. Successful replication suggests that EV holds promise as a construct with clinical utility for early interventions attempting to improve developmental outcomes in children from poor families. (Contains 8 tables and 2 notes.)