Hypokalemia, defined as a less than 3.5 mmol/I serum potassium, is probably the most common electrolyte abnormality in clinical practice. It is usually well tolerated but, if severe, it can be life-threatening. However, even mild or moderate hypokalemia increases the risks of morbidity and mortality in patients with cardiovascular disease. When hypokalemia is identified, the underlying cause should be established and the disorder immediately treated.