The Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency disease, arising from mutations of the WAS-protein (WASP) gene. Previously, we have reported that mononuclear cells from WAS patients showed lack/reduced of the intracellular WASP (WASP dim) by flow cytometric analysis, and analysis of WASP by flow cytometry (FCM-WASP) was useful for WAS diagnosis. In this study, we report a WAS patient who showed the unique pattern of FCM-WASP. The patient had the small population of normal expression of WASP (WASP bright) mononuclear cells together with the major WASP dim population. The WASP bright cells were detected in T cells, not in B cells or in monocytes. Surprisingly, the molecular studies of the WASP bright cells revealed that the inherited mutation of WASP gene was reversed to normal. His mother was proved as a WAS carrier, and HLA studies and microsatellite polymorphic studies proved that the WASP bright cells were derived from the patient himself. Therefore, we concluded that the WASP bright cells were resulted from spontaneous in vivo reversion of the inherited mutation. Furthermore, the scanning electron microscopic studies indicated that WASP-positive cells from the patient restored the dense microvillus surface projections that were hardly observed in the WASP dim cells. This case might have significant implications regarding the prospects of the future gene therapy for WAS patients.