Spurious elevation of the serum potassium can be seen if the platelet count exceeds 1,000 x 10(9)/L in patients with myeloproliferative disease. To see if serum potassium is increased at more modest elevations of the platelet count we studied these parameters in 283 controls and 161 patients with reactive thrombocytosis. The incidence of hyperkalemia was 34% in patients with over 500 x 10(9) platelets/L compared with 9% if the platelet count was below 250 x 10(9)/L. Over this range there was a significant correlation of potassium increasing with the platelet count. The higher potassium values could not be explained on the basis of leukocytosis, renal insufficiency, or acidosis, suggesting that the elevated serum levels were often spurious. In support of this conclusion, the serum potassium exceeded plasma potassium by a larger amount in patients with thrombocytosis. We conclude that serum potassium rises in direct proportion to the platelet count in normal patients and in those with thrombocytosis, and that this increment is an artifact.