Disturbances in calcium-phosphate homeostasis are common after kidney transplantation. We aimed to assess the relationship between deregulations in plasma calcium and phosphate over time and mortality and death-censored graft failure (DCGF). In this prospective cohort study we included kidney transplant recipients with ≥2 plasma calcium and phosphate measurements. Data were analyzed using time-updated Cox regression analyses adjusted for potential confounders including time-updated kidney function. We included 2,769 patients (mean age 47±14 years, 42.3% female) with 138,496 plasma calcium and phosphate levels (median [IQR] 43 [31-61] measurements per patient). During follow-up of 16.3 [8.7-25.2] years, 17.2% developed DCGF and 7.9% died. Post-transplant hypercalcemia was associated with an increased risk of mortality (1.63 [1.31-2.00], P<0.0001), but not DCGF. Hyperphosphatemia was associated with both DCGF (2.59 [2.05-3.27], P<0.0001) and mortality (3.14 [2.58-3.82], P<0.0001). Only the association between hypercalcemia and mortality remained significant in sensitivity analyses censored by a simultaneous eGFR <45 mL/min/1.73 m2 . Hypocalcemia and hypophosphatemia were not consistently associated with either outcome. Post-transplant hypercalcemia, even in the presence of preserved kidney function, was associated with an increased mortality risk. Associations of hyperphosphatemia with DCGF and mortality may be driven by eGFR.