We describe a 9-year-old boy with Wiskott-Aldrich syndrome and IgM-rheumatoid factor-positive arthritis who presented expansion of two distinct subsets (one CD8dim and the other CD8-) of large granular lymphocytes. Natural killer activity against the K-562 cell line was absent. An increased percentage of CD5+ B cells was also observed. Since patients with Wiskott-Aldrich syndrome are at risk of developing autoimmune disorders - conditions in which increased CD5+ B cells have been observed - the high percentage of CD5+ B cells together with the presence of IgM-rheumatoid factor and anti-platelet antibodies may represent an early manifestation of an autoimmune process. The possible relationship between CD5+ B cells and large granular lymphocyte expansion is discussed.