Wiskott-Aldrich syndrome (WAS) is an X-linked recessive disease caused by mutations in the recently identified WAS protein gene (WASP). In some X-linked genetic disorders skewed X-inactivation has been observed in all cell populations or some specific cell lineages of female carriers. Recently, female carriers of WAS were also revealed to present skewed X-inactivation patterns at the haemopoietic stem cell level. However, it is not clear if all haematological cells expressing the mutant WASP allele are eliminated in WAS carriers. By reverse transcription PCR methods, we studied 14 WAS carriers from 10 different families to assess whether blood cells expressing the mutant WASP message were present in their peripheral blood. The mutations of each WAS patient were known and carrier diagnosis of their female family members was performed using specific mutation analysis. We detected circulating lymphocytes and granulocytes expressing the mutant WASP message in most of the WAS carriers, nevertheless they showed skewed X-chromosomal inactivation patterns. Interestingly, the presence of blood cells expressing the mutant WASP message seemed to correlate to the WASP genotype and the age of the carriers.