ObjectiveTo investigate the relationship between admission serum potassium level and long-term prognosis in patients with unstable angina. MethodsWe studied the data of 1 412 patients with unstable angina who received coronary angiography examinations and completed the follow-up between July 2008 and September 2012. Serum potassium level within the first 24 hours after admission was collected. According to the serum potassium level, the patients were divided into three groups:those with a serum potassium level lower than 3.5 mmol/L, those with a level between 3.5 and 5.0 mmol/L and those with a level higher than 5.0 mmol/L. Then, we analyzed the relationship between admission serum potassium level and long-term prognosis in patients with unstable angina. ResultsThere was a U-shaped relationship between admission serum potassium level and long-term mortality that persisted after multivariable adjustment in patients with unstable angina. The all-cause mortality risk was the lowest in the group of patients with a potassium level of 3.5 to 5.0 mmol/L, whereas mortality was higher in patients with potassium level lower than 3.5 mmol/L and higher than 5.0 mmol/L [HR=1.89, 95%CI (1.13, 3.17), P=0.016; HR=1.64, 95%CI (0.40, 6.77), P=0.493]. Compared with patients with a serum potassium level between 3.5 and 5.0 mmol/L, the cardiovascular mortality risk was significantly higher in those patients with a potassium level lower than 3.5 mmol/L [HR=1.99, 95%CI (1.01, 3.94), P=0.048]. ConclusionThere is a U-shaped relationship between admission serum potassium level and long-term all-cause mortality rate, and the all-cause mortality rate and cardiovascular mortality risk was the lowest in patients with a potassium level between 3.5 and 5.0 mmol/L.