Splenic lymphoma with villous lymphocytes (SLVL) is a low-grade disorder that regularly presents with peripheral blood involvement. We describe the immunophenotype of the circulating cells from 100 SLVL patients whose disease has been characterized on clinical, morphologic, and histologic grounds. Cells from all cases expressed B-cell antigens (CD19 and CD37) and/or HLA-Dr and showed light chain restriction (kappa/lambda: 1.5/1) with moderate to strong intensity of membrane Ig staining. Cells from most cases (> 80%) were CD24+, FMC7+, and expressed strongly membrane CD22. The monoclonal antibodies CD10, CD23, and CD38 were positive in one-third of the cases; CD11c in 47%; and CD25 in 25% of cases. A minority of cases (< 20%) were positive with HC2, B-ly-7, and CD5. However, none of the 19 CD5+ cases had the phenotype characteristic of chronic lymphocytic leukemia (CD5+, CD23+, FMC7-, weak surface Ig and membrane CD22). None of the 17 CD25+ cases had the immunophenotype typical of hairy cell leukemia (CD25+, CD11c+, HC2+, B-ly-7+). HC2 and B-ly-7 were the most useful reagents to distinguish SLVL from hairy cell leukemia. Our findings demonstrate that SLVL has a distinct immunologic profile and that monoclonal antibodies are important for the differential diagnosis between this disease and other B-lymphoproliferative disorders with which SLVL can be confused.