New-onset diabetes after transplantation (NODAT) is a frequent complication after liver transplantation and has a negative impact on both patient and graft survival. In analogy with the previous finding of an association between posttransplant hypomagnesemia and NODAT in renal transplant recipients, the relation between both pretransplant and posttransplant hypomagnesemia and NODAT was studied in liver transplant recipients (LTRs). One hundred sixty-nine adult LTRs (>18 years old) without diabetes who underwent transplantation between 2004 and 2009 were studied (mean age = 52.11 ± 12.6 years, proportion of LTRs who were male = 67.5%, body mass index = 25.5 ± 4.4 kg/m², proportion receiving tacrolimus = 90.0%). NODAT was defined according to the American Diabetes Association criteria. The association of NODAT with both pretransplant and posttransplant serum magnesium (Mg) was examined. Overall, 52 of 169 patients (30.8%) developed NODAT, and 57.7% of these (30 patients) were treated with antidiabetic drugs. Both pretransplant Mg levels and Mg levels in the first month after transplantation were lower in patients developing NODAT (P = 0.008 and P = 0.001, respectively). A multivariate regression model (adjusted for weight, pretransplant glucose levels, hyperglycemia in the first week after transplantation, gender, hepatitis C, and corticosteroid dosing) demonstrated both pretransplant Mg levels (hazard ratio = 0.844 per 0.1 mg/dL increase, 95% confidence interval = 0.764-0.932, P = 0.001) and posttransplant Mg levels (hazard ratio = 0.659, 95% confidence interval = 0.518-0.838, P = 0.001) to be independent predictors of NODAT together with age, biopsy-proven acute rejection, and cytomegalovirus (CMV) infection in the first year after transplantation. In conclusion, pretransplant hypomagnesemia and early posttransplant hypomagnesemia are independent predictors of new-onset diabetes after liver transplantation. Other risk factors are age, biopsy-proven acute rejection, and CMV infection.