A growing evidence base supports early intensive behavioral intervention (EIBI) for young children with autism spectrum disorder. We have found only one study exploring the outcome of EIBI for children with Rett syndrome, which reported little effect. It suggested that future studies should employ more fine-grained outcome measures. We provided EIBI for twin 3-year-old girls with Rett syndrome for a period of 3 years. We analyzed raw scores and standard scores from a measure of adaptive behavior and a detailed assessment of skills across 25 areas. We detected moderate but clinically meaningful gains in areas such as communication and self-help. Gains are discussed from a quality of life perspective and whether the moderate effects can justify the resources required in EIBI.