Rett syndrome is a progressive neurodevelopmental disorder with a well-defined clinical spectrum and course. Recently, mutations in the gene encoding X-linked methyl-CpG binding protein 2 (MECP2) have been identified as the cause of Rett syndrome. Along with the classic form, variant forms of Rett syndrome and Rett syndrome phenotypes are also recognized. We report on a girl who, at age 2 months, developed an acute encephalopathy with destructive brain damage 12 hours after acellular pertussis vaccination. Peripheral lymphocyte subset analysis revealed the existence of T lymphocytes double positive for CD4 and CD8 markers. This pattern normalized over the following 3 months. Months later, the girl manifested a Rett syndrome phenotype. DNA screening of the MECP2 gene was unrevealing in the child and her parents. This previously unreported association emphasizes the notion that Rett syndrome phenotypes can result from different (either genetic or environmental) causes.