Pseudohyperkalemia, a rise in serum potassium concentration with concurrently normal plasma potassium concentration, is an in vitro phenomenon that was first described 50 years ago. It was originally attributed to the release of potassium from platelets during platelet aggregation and degranulation, and a significant correlation between pseudohyperkalemia and platelet count was established. During the last decade, new data were added to this phenomenon. In particular, pseudohyperkalemia was defined when serum potassium concentration exceeded that of plasma by more than 0.4 mmol/L provided that samples are collected under strict techniques, remain at room temperature and are tested within 1 hour from blood specimen collection. Moreover, it is positively correlated to (1) thrombocytosis due to the release of potassium from platelet granules during coagulation, (2) erythrocytosis due to the dilution of the released potassium in smaller volumes of serum, and (3) the presence of activated platelets, which have the capability of aggregation at a higher speed and release more potassium during degranulation. However, pseudohyperkalemia may be "masked" when in a state of hypokalemia because potassium moves back into the intracellular space in vitro, and the phenomenon is ameliorated or even not detected.