The Wiskott-Aldrich syndrome (WAS) is an X-linked recessive genetic disease in which the basic molecular defect is unknown. We previously located the WAS gene between two DNA markers, DXS7 (Xp11.3) and DXS14 (Xp11), and mapped it to the proximal short arm of the human X chromosome (Kwan et al., 1988, Genomics 3:39-43). In this study, further mapping was performed on 17 WAS families with two additional RFLP markers, TIMP and DXS255. Our data suggest that DXS255 is closer to the WAS locus than any other markers that have been previously described, with a multipoint maximum lod score of Z = 8.59 at 1.2 cM distal to DXS255 and thus further refine the position of the WAS gene on the short arm of the X chromosome. Possible locations for the WAS gene are entirely confined between TIMP (Xp11.3) and DXS255 (Xp11.22). Use of these markers thus represents a major improvement in genetic prediction in WAS families.