Background: Abnormal serum magnesium (Mg) concentrations are common and associated with worse mortality in kidney-transplant recipients. Many kidney and transplant-related factors affect Mg homeostasis. The concentration of the active form, ionized Mg (iMg), is not measured clinically, and total Mg (tMg) and iMg correlations have conflicted.We hypothesized that iMg and tMg concentrations show poor categorical agreement (i.e., low, normal, and high) in kidney-transplant recipients but that ionized calcium (iCa) correlates with iMg. Methods: We retrospectively evaluated hypomagnesemia in kidney-transplant recipients over a 2-yr period. We prospectively collected blood at 0–28 days post-transplant to measure correlations between iMg and iCa/tMg. iMg and iCa concentrations in the reference ranges of 0.44–0.65 and 1.0–1.3 mmol/L, respectively, were considered normal. Fisher’s exact test and unweighted kappa statistics revealed category agreements. Pearson’s correlation coefficients and linear regression measured correlations. Results: Among 58 retrospective kidney-transplant recipients, 54 (93%) had tMg < 0.66 mmol/L, 28/58 (48%) received Mg supplementation, and 20/28 (71%) had tacrolimus dose adjustments during supplementation. In 13 prospective transplant recipients (N = 43 samples), iMg and tMg showed strong category agreement (P = 0.0003) and correlation (r = 0.71, P < 0.001), whereas iMg and iCa did not (P = 0.7; r = –0.25, P = 0.103, respectively). Conclusions tMg and iMg exhibited strong correlation following kidney transplantation.However, iCa may not be an accurate surrogate for iMg. Determining the effect of Mg supplementation and the Mg concentration where supplementation is clinically necessary are important next steps.