We report on an interstitial duplication of the long arm of chromosome 11 [46XX,dup(11) (q23.3)] in a girl with atypical Rett syndrome (RS). This case was discovered during a systematic cytogenetic study of RS. Fluorescent in situ hybridization including total chromosome painting and use of regional specific YAC, cosmid and plasmid probes, was used to confirm the chromosome 11q involvement and to identify the landmarks of the smallest 11q duplication reported to date. The findings are compared to cases of trisomy 11q reported previously, all of which have a larger duplication and different clinical manifestations. Surprisingly, mental retardation and behavior disorders are less severe in these cases.