We describe four female patients with primary splenic low-grade non-Hodgkin's B-cell lymphomas with the morphology and immunophenotype of splenic marginal zone lymphocytes. The patients presented with splenomegaly, anemia, and weight loss. The bone marrow was involved in all four cases. Liver involvement was found in one patient; and in another, a CT scan revealed lymphadenopathy in the chest and abdomen. The histology of the spleen was characterized by broad concentric strands of monomorphic medium-sized lymphocytes around lymphoid follicles in one case and infiltrating follicles in two cases. Selective replacement of follicles was seen in one case. Tumor in splenic hilar lymph nodes (four cases) and liver (one case) was similar. Three patients remain well 4, 9, and 12 months, respectively, after splenectomy without further treatment. One patient who received chemotherapy died 1 year after splenectomy.