The Early Communication Indicator (ECI) is a progress monitoring measure designed to support intervention decisions of the home visitors and early educators who serve infants and toddlers. The present study sought to add to the criterion validity claims of the ECI in a large sample of children using measures of language and preliteracy not previously investigated. Early Head Start service providers administered and scored ECIs quarterly for infants and toddlers in their caseload as part of standard services. In addition, a battery of language and early literacy criterion tests were administered by researchers when children were 12, 24, 36, and 48 months of age. Analyses of this longitudinal data then examined concurrent and predictive correlational patterns. Results indicated that children grew in communicative proficiency with age, and weak to moderately strong patterns of relationship emerged that differed by ECI scale, age, and criterion measure. The strongest positive patterns of relationships were between Single Words and Multiple Words and the criterion at older ages. Gestures and Vocalizations established a pattern of negative relationships to the criterion measures. Implications for research and practice are discussed. [This article was published in "Assessment for Effective Intervention" (EJ1263844).]