The neurologic disorder Rett syndrome was originally described exclusively in girls. We present two boys with clinical features of Rett syndrome. Other than head circumference deceleration, no longer considered mandatory, patient 1 meets all of the criteria. Using fluorescent in situ hybridization analysis, 97.6% of cells were found to be karyotypically normal (46,XY). No mutation was detected on screening of the coding region of the MECP2 gene. The second patient also has classic features of Rett syndrome. However, cytogenetic analysis of peripheral blood revealed a karyotype 47,XXY/46,XY confirming mosaicism for Klinefelter's syndrome. A T158M missense mutation in the methylcytosine-binding domain of the MECP2 gene was identified. A diagnostic bias against the clinical identification of Rett syndrome in boys may exist. This presentation of the male phenotype could be more common than it would appear, although boys with MECP2 mutations might also manifest in other ways. Rett syndrome remains a clinical diagnosis that should not be dismissed in boys, and thorough evaluation including karyotype and mutation testing is warranted.