<h3>Purpose</h3> The TEAMMATE Trial is studying everolimus (EVL) with low-dose tacrolimus (LDTAC) vs. standard tacrolimus (TAC) with mycophenolate (MMF) in a randomized trial in children enrolled at 6 months post-heart transplant. Since therapeutic levels are correlated with lower rejection rates, we sought to describe the immunosuppressant drug level monitoring experience in the trial to date. <h3>Methods</h3> All children in the TEAMMATE Trial remaining on assigned therapy as of August 2021 were included. Drug levels were obtained at 0, 3, 6, 9, 12, 18, 24, & 30 months post-randomization. Formulation, time to therapeutic level, and number of dose adjustments were tracked at each study visit. Stability was assessed by the coefficient of variation (CoV) (SD/mean). Adherence was assessed by the % of levels within range; and the SD of levels divided by therapeutic window (lower values indicating better adherence). Independence of all levels was assumed in analysis. <h3>Results</h3> A total of 211 children were randomized to either EVL/LDTAC (n=107) or usual care TAC/MMF (n=104). After randomization, a therapeutic level was achieved in 94% (EVL) & 91% (MMF). Liquid formulation was used in 37% of EVL subjects. Median time to a therapeutic level for EVL was 10 (tablet) & 11 (liquid) days. The CoV was lower for TAC (36%), EVL (47%), & LDTAC (52%), compared to MMF (103%). Drug levels were in target range 80% (EVL), 20% (MMF), 55% (LDTAC), & 54% (TAC) of the time (<b>Figure</b>). The SD of levels divided by therapeutic window was lowest for EVL (44%), compared to TAC (94%), LDTAC (112%), & MMF (158%). In response to these levels, dose changes were made in 13% (EVL) & 18% (LDTAC) in the EVL/LDTAC group (16% combined) and in 27% (TAC) and 11% (MMF) in the control group (22% combined). <h3>Conclusion</h3> EVL achieves stable target levels more frequently than MMF, LDTAC or TAC, likely due to decreased trough level variability and its wider therapeutic window. Fewer dose adjustments have been required for the EVL/LDTAC regimen compared to TAC/MMF.