Splenic marginal zone lymphoma (SMZL) is a low-grade primary splenic B cell lymphoma, originally thought to be related to splenic marginal zone B cells. Later studies showed that SMZL sometimes may be accompanied by villous lymphocytes in the peripheral blood, a condition previously characterized as splenic lymphoma with villous lymphocytes (SLVL). The relationship between SMZL and splenic marginal zone B cells has recently been called into question. We report four further cases of SMZL, two of which were associated with villous lymphocytes in the peripheral blood. In addition to immunophenotypical analysis, we have studied the IgV(H) genes in each case, because the extent and patterns of their mutation can indicate the normal B cell counterpart of lymphomas. The IgV(H) genes in the four cases of SMZL studied are mutated, which is consistent with their origin from postfollicular marginal zone B cells. Evidence of ongoing mutation was also observed. This contrasts with a study showing that blood-borne tumor cells in SLVL show no sign of ongoing mutation. It is possible that the ongoing mutations in the cases studied here are acquired in a splenic microenvironment, such as that found in the follicle center.