What is known and objective: Tacrolimus (TAC) is an immunosuppressant with large interpatient pharmacokinetic variability and a narrow therapeutic index. We report a case of acute cellular rejection (ACR) type IB with insufficient TAC blood concentrations (TAC C0). Case summary: ACR developed on the eighth postoperative day of kidney transplantation. During this period, TAC C0 were insufficient. This referred pharmacogenetic assessment disclosed the patient as a CYP3A5 expresser and CYP3A4*1B carrier. According to the genotype, higher doses of TAC, 15 mg twice daily, were administered and targeted TAC C0 were achieved. What is new and Conclusion: Our case presents an association of TAC rapid clearance and two alleles modifying greater CYP3A enzyme activity.