Tacrolimus, a cornerstone immunosuppressant, is available as a twice-daily formulation (tacrolimus bid). A once-daily prolonged-release formulation (tacrolimus qd) has been developed. This 6-week, randomized, phase 2, multicenter, open-label, prospective trial in primary liver transplant recipients investigated and compared the pharmacokinetics (PK) of tacrolimus for qd and bid formulations. All patients received tacrolimus-based immunosuppression (tacrolimus qd, n ¼ 67; bid, n ¼ 62). PK data were available for 77 patients (tacrolimus qd, n ¼ 45; bid, n ¼ 32). Tacrolimus area under the curve (AUC) from 0 to 24 hours (AUC 0-24 ) at equivalent doses was approximately 50% lower for tacrolimus qd than for bid on day 1 (146 versus 264 ngÁh/mL, respectively), but by day 14 was comparable between treatments (324 and 287 ngÁh/mL, respectively) with higher tacrolimus qd doses. There was a strong correlation between AUC 0-24 and concentration at 24 hours for tacrolimus qd and bid (r ¼ 0.92 and r ¼ 0.76, respectively). Furthermore, the relationship between these 2 parameters (ie, the slope of the line) was also similar for the 2 formulations. Efficacy endpoints were comparable for both formulations at 6 weeks with Abbreviations: AE, adverse event; AUC 0-24 , area under the curve from 0 to 24 hours; AZA, azathioprine; bid, twice daily; BPAR, biopsy-proven acute rejection; C 24 , concentration at 24 hours; C max , maximum concentration; C min , minimum blood concentration; MMF, mycophenolate mofetil; NEC, not elsewhere classified; PK, pharmacokinetics; qd, once daily; SD, standard deviation; T max , time to maximum concentration. This study and statistical analyses, and the editorial and project management services of Acumed involved in the preparation of this manuscript, were supported by Astellas Pharma Europe, Ltd., Staines, UK. L. Fischer, B. Gridelli, A. Roy, A. Vitale, A. Valdivieso, E. Varo, D. Seehofer, B. Ericzon, and K. Boudjema have declared no conflicts of interest. P. Trunečka has acted as study investigator and advisory board member for Astellas and has received speaker's fees. S. Lynch has received honoraria for chairing and speaking at international meetings for Astellas and JanssenCilag,