Pseudohyperkalaemia in thrombocythaemia, in its primary form or associated with polycythaemia vera, can occur with platelet counts in excess of 600 X 10(9)/l, and it is not related to high leukocyte or thrombocyte counts. An increment of 0.15 mmol/l serum potassium was found for every 100 X 10(9)/l rise in platelet count with a correlation coefficient of 0.82. Potassium from platelets is not released during the aggregation phase but during the degranulation phase of the coagulation process.