The Wiskott-Aldrich syndrome (WAS) is a severe X-linked, recessive disorder, with a high mortality rate at early age due to hemorrhages, infections, and lymphoid malignancies. The molecular pathogenesis of the disease is unknown. Carrier females of WAS are clinically and immunologically normal, thus precluding carrier detection by simple laboratory tests. Major advances in molecular genetics have allowed mapping of the WAS gene to the pericentromeric short arm of the X chromosome, and have made carrier detection and prenatal diagnosis feasible by segregation analysis with closely linked polymorphic DNA markers. Furthermore, the observation that carriers of WAS exhibit a unilateral inactivation of the X chromosome in hematopoietic cells has provided a new tool for carrier detection. However, critical interpretation of molecular analysis data is essential to provide accurate genetic counseling to WAS families.