The aim of this study was to determine the frequency, timing, and associated features of developmental regression in MECP2 duplication syndrome. We also examined whether duplication size was associated with regression. Comprehensive psychological evaluations were used to assess 17 boys with MECP2 duplication syndrome. Information about regression was gathered via parent report. Eight of 17 boys exhibited regression in language skills, while seven of 17 exhibited regression in other skill areas. Regression in "other skill" areas coincided with seizure onset and with a prior autism diagnosis in six of seven participants. Regression was not associated with duplication size. Questions remain as to why some boys regress, and future work is necessary to understand the underlying mechanism(s) that causes regression.