The mechanism responsible for the decreased extra-renal CYP3A activity in chronic kidney disease (CKD) patients remains unknown. Using an animal model, we previously found that elevated levels of serum intact parathyroid hormone (iPTH) caused a reduced CYP3A activity. This retrospective observational study assessed the relationship between serum iPTH levels and the blood concentration or dosage of tacrolimus, a CYP3A substrate, after oral administration in kidney transplant patients. Thirty-four patients were enrolled who had kidney transplants between April 2014 and March 2016 and who had been administrated once- daily prolonged-release tacrolimus (Graceptor^®, Astellas Pharm Inc.). Among the 34 patients, 22 had taken a CYP3A inhibitor. These patients were excluded from the study. A significant positive correlation between serum iPTH and tacrolimus trough levels was found at 4 d before kidney transplantation in 12 patients who were not receiving potent CYP3A inhibitor. In addition, serum iPTH levels before transplantation could serve as a factor for the dose of tacrolimus up to 1 year after transplantation. Monitoring serum iPTH levels could predict the trough level for the initial administration of tacrolimus, and may serve as an index for the initial dose of tacrolimus in kidney transplantation patients.