A male aged 64 who had been treated for polycythemia vera for seven years underwent gastrectomy, splenectomy and partial hepatectomy. The preoperative hematocrit and platelet count was considered to be on the margin of operable condition. However, it was necessary to perform the splenectomy. This caused on increase in the platelet count as was expected. The patient was postoperatively given gabexate mesilate and heparin sodium intravenously, and hydroxycarbamide and ticlopidine hydrochloride orally to prevent DIC. This treatment was carried out successfully and platelet counts returned normal. Pseudohyperkalemia was seen in this case, and platelet counts and time after blood sampling correlated with serum potassium level and with the difference between serum and plasma potassium levels.