The Early Communication Indicator (ECI) is a measure for universal screening, intervention decision-making, progress monitoring for infants and toddlers needing higher levels of support, and program accountability. In the context of the ECI's long-term wide-scale use for these purposes, we examined the invariance of ECI measurement in two samples of the same Early Head Start (EHS) population differing in the years data were collected. Invariance or equivalence across samples is an important step in measurement validation because making inferences assumes that the measurements are factorially invariant. A number of time-covarying factors (e.g., assessors, children, etc.) can be hypothesized as threats to measurement invariance. Results of latent growth curve analyses indicated similarity in the functional forms (velocity and shape) of the ECIs four key skill trajectories between groups of children and ECI vocalizations, single, and multiple words trajectories met strong factorial and structural invariance. Gestures met only weak factorial invariance. ECI total communications, a weighted composite of the four scales, also met both strong factorial and structural invariance. With one exception, results indicated that the ECI produced comparable growth estimates over different conditions of programs, assessors, and children over time, strengthening the construct validity of the ECI. Implications are discussed. (author abstract)