The Early Communication Indicator (ECI) was designed to measure expressive communication progress in young children. We evaluated using the 6-min ECI procedure for a new purpose—a sampling context for stable measures of vocal development of young children with autism spectrum disorder (ASD). We evaluated how many ECI sessions were required to adequately stabilize estimates of volubility, communicative use, and phonological complexity of vocalizations at two periods (average of 10 months apart). Participants included 83 young children with ASD (M age = 23.33 months). At study initiation, two phonological complexity variables required two sessions; other variables required three. At study endpoint, all variables required fewer sessions. Findings support the feasibility and stability of using the ECI for the new purpose.