Treatment-related changes in plasma von Willebrand factor (vWf) multimers were studied in two patients with thrombotic thrombocytopenic purpura (TTP) undergoing splenectomy because of refractoriness to plasma exchange. In both cases, a decrease in the largest normal vWf multimers was found at presentation. In one case, splenectomy resulted in a long-term remission and was followed by a normal multimeric pattern. In the other case, splenectomy failed to produce a remission and was followed by the appearance of ultralarge multimers, which disappeared--resulting in a normal pattern--when a sustained remission was achieved after vincristine infusion and further plasma exchanges. These results suggest that splenectomy per se has no consistent effect on the vWf multimeric structure in TTP patients, and provide further evidence on the good prognostic value of the achievement of a normal multimeric pattern in these patients.