We describe a 10-year-old girl with features of a penta-X syndrome. Cytogenetic analysis revealed a 49,XXXXX karyotype and molecular analysis of X-linked polymorphic markers showed that this aneuploidy arose by successive maternal non disjunctions. Apart from these features the patient has a lifelong history of eczema, recurrent pneumonia, and staphylococcal abscesses. Together with consistently increased serum IgE levels, low antibody responses, and low levels of serum IgA and IgG2, these findings are characteristic for the hyper IgE syndrome. While pentasomy X may be due to sequential non disjunctions in meiosis I and meiosis II in the mother, the underlying pathomechanism in hyper IgE syndrome remains unclear. This case is the first with co-existence of pentasomy X and hyper IgE syndromes.