A Study to Compare How the Body Absorbs and Processes Two Different Formulations of the Anti-rejection Medication Tacrolimus (Advagraf® or Prograf®) in Children Receiving an Organ Transplant, and How Safe and Effective They Are Over a Longer Period of Time

Participants undergoing primary heart, kidney or liver transplantation and meeting the Inclusion Criteria and complying with the Exclusion Criteria prior to initiation of tacrolimus therapy will be enrolled. Participants will be randomized to treatment with either Advagraf® or Prograf®. The randomization will be on a 1:1 basis stratified by organ and centre. The study is divided in to two parts: Part A: The initial pharmacokinetic part of the study. Part B: A long term follow-up of one year. The main objective of Part A of the study is to collect PK data following administration of Advagraf® and Prograf® in de novo pediatric allograft recipients. Part B allows comparison of the safety and efficacy profiles of Advagraf® vs. Prograf® for longer term (52 weeks) post allograft transplantation. Part C: Continuation of long-term follow-up (from Day 365 onwards). Participants who have completed Part B and to whom continued treatment with Advagraf® is not currently available, will be offered participation in a continuation of long-term follow-up Part C. Part C will continue until Advagraf® becomes available to these participants or these participants' discontinuation, whichever is the earliest. This applies to participants in the following countries: Czech Republic, Italy, UK and Poland only.