The neurodevelopmental disorder Rett syndrome (RTT) is caused by sporadic mutations in the transcriptional factor methyl-CpG-binding protein 2 (MeCP2). Although it is thought that the primary cause of RTT is cell autonomous, resulting from a lack of functional MeCP2 in neurons, whether non-cell autonomous factors contribute to the disease is unknown. We found that the loss of MeCP2 occurs not only in neurons but also in glial cells of RTT brains. Using an in vitro co-culture system, we found that mutant astrocytes from a RTT mouse model, and their conditioned medium, failed to support normal dendritic morphology of either wild-type or mutant hippocampal neurons. Our studies suggest that astrocytes in the RTT brain carrying MeCP2 mutations have a non-cell autonomous effect on neuronal properties, probably as a result of aberrant secretion of soluble factor(s).