A Saudi Arab boy presented in early childhood with thrombocytopenia, morphologically large and normal sized platelets, increased mean platelet volume, and a hypermegakaryocytic bone marrow. There was no clinical and laboratory evidence of any significant immunological abnormalities. Similar findings in two other brothers suggested strongly that they were all suffering from an X linked recessive thrombocytopenic disorder. Results of DNA analysis with the probe M27 beta are consistent with X linkage and indicate also that the locus of the relevant gene lies close to or is identical to the locus of the gene for the Wiskott-Aldrich syndrome (WAS). Because of various features which include the presence of large and normal sized platelets (rather than small platelets) and freedom from significant immune deficiencies, it is likely that the X linked recessive thrombocytopenia in this family is an isolated entity quite distinct from the classical WAS phenotype. However, a modified expression of the WAS gene producing a mild phenotypic variant cannot be excluded entirely.