Hereditary X-linked thrombocytopenia occurs either as isolated thrombocytopenia or as a part of the Wiskott-Aldrich syndrome (WAS). We studied X-linked thrombocytopenia in a family with eight affected male members, none of whom exhibited the increased susceptibility to infection that occurs in WAS. We found a significant linkage between thrombocytopenia and DXS 146, a marker on the proximal part of the short arm of the X-chromosome. WAS has previously been mapped to the same chromosomal region. The present findings indicate that X-linked thrombocytopenia and WAS are closely related and may even be caused by different mutations of the same gene. This view is supported by our findings of atopic symptoms and minor deviations in immunologic variables among some of the affected subjects.