Background. Potassium plays a key role in human metabolism in both health and disease. The impact of recipient serum potassium concentration [K] on mortality after liver transplantation has not been described previously. Methods. We assessed the effect of recipient [K] on the survival of adult first single-organ liver transplant recipients in the United Kingdom and Ireland between March 1, 1994, and February 28, 2007 (n=5942), adjusting for recipient, donor, and graft characteristics. Results. The overall risk-adjusted mortality significantly varied by [K], being higher among hyperkalemic ([K]>5.0 mmol/L) recipients (n=424, hazard ratio [HR] 1.38, 95% confidence interval [CI] 1.01-1.88) and those with [K] of 4.5-5.0 mmol/L (n=1154, HR 1.47, 95% CI 1.13-1.91), compared with hypokalemic ([K]<3.5 mmol/L) recipients (n=360). However, the excess mortality was confined to the first posttransplant year among hyperkalemic recipients (HR 1.61,95% CI 1.10-2.35) with no significant difference thereafter (HR 1.03, 95% CI 0.62-1.73). This was also true for recipients with [K] of 4.5 to 5.0 mmol/L (1 year: HR 1.09,95% CI 0.71-1.66). In contrast, those with [K] of 3.5 to 3.9 mmol/L (n= 1518) and [K] of 4.0-4.4 mmol/L (n=2091) had similar risk-adjusted mortality at the above time points. When [K] was used as a continuous variable in the multivariable analysis, a mmol increase in [K] was associated with an increased adjusted risk of mortality of 27% (95% CI 12%-44%) at 1 year and 19% (95% CI 7%-31%) at 5 years. Conclusion. Recipient [K] is an independent predictor of death after liver transplantation. This finding could be of clinical utility in the management, risk stratification, selection, and prioritization of appropriate candidates for transplantation among patients with end-stage liver disease.