Rett Syndrome (RTT) is a severe genetic form of autism that mainly affects girls. Children with RTT also develop other symptoms such as impaired ability to move and seizures. A common feature of RTT is abnormal breathing and heart rate patterns that can cause sudden death. There is no known treatment for RTT. The investigators propose to evaluate the efficacy of IGF1 in reversing the autonomic and neurodevelopmental features of RTT. In a randomized two-arm, double-blind, placebo-controlled crossover clinical trial, patients with RTT who have been diagnosed according to established clinical criteria and who have known MECP2 mutations will be enrolled and randomized to receive either IGF1 twice daily for 5 months or placebo. A crossover design will be used to insure that all study patients will have access to the trial medication and will allow for a secondary analysis of changes in the “off treatment” group. Primary outcome measures will be improvement in cardiorespiratory dysregulation and neurodevelopment. The study will test the effectiveness and safety of rhIGF1 in a group of 30 children with RTT treated with rhIGF and placebo for 20 weeks. To prevent biases, caregivers and the study investigators will not know when the subjects are receiving the drug or placebo. The safety of rhIGF1 will be evaluated and the investigators will perform a range of measurements of specific symptoms of RTT, including non-invasive measurements of the regulation of breathing and heart. They will compare how well the subjects' cardiorespiratory regulation and neurodevelopmental function respond to rhIGF1 treatment. This study will provide important information on the safety and efficacy of rhIGF1 in improving symptoms in RTT and potentially lead to future clinical trials using rhIGF1 as a strategy to treat other ASDs.