The present study explored the association between the dosage level of Part C Early Intervention (EI) services and growth in adaptive behavior from early childhood through adolescence for children with developmental disabilities. Children who received more hours of service than expected based on their characteristics and those of their family measured at EI intake had greater skills in communication, socialization, and daily living skills when they ended EI services at EI. In addition, children who received more hours of service showed greater improvements in all three domains of adaptive behavior over time. The short- and long-term benefits of higher dosage levels of EI services suggest that Part C EI practices and policies should be geared toward increasing service hours, either by increasing the number of hours of scheduled service or minimizing disruptions to scheduled service.