Within few years the peritoneal membrane of adult peritoneal dialysis (PD) patients undergoes substantial morphological transformation, including progressive fibrosis, vasculopathy and neoangiogenesis. Ultrafiltration capacity steadily declines and ultimately results in PD failure. In children, peritoneal biopsies demonstrating PD associated alterations have not yet been obtained. They, however, should be particularly informative, since secondary tissue and vascular pathology related to ageing or diabetes is absent. An international, prospective peritoneal membrane biopsy study in children on PD will therefore be performed. Biopsies will be obtained at time of PD catheter insertion, on occasion of intercurrent abdominal surgery (e.g. hernia repair, catheter exchange) and at time of renal transplantation. Quantitative histomorphometry and tissue protein expression analyses will be correlated with time integrated PD treatment modalities and functional characteristics as well as inflammatory and cardiovascular comorbidity surrogate parameter. Blood will be obtained during clinical routine sampling. Biopsies will be obtained during clinically indicated operations, without substantially increasing operation time and associated surgical risks. The detailed histomorphometry of the PD membrane will give additional information, potentially impacting on the individual PD regime. 3/2018: The analyses of the pediatric PD biopsy demonstrated early and major transformation of the peritoneal membrane with neutral pH low GDP fluids, and significant vasculopathy already in children with CKD stage 5, further progressing with PD. The underlying mechanisms are partly understood, only. In view of these major findings and the numerous open questions, collection of biosamples will be continued in children and also in adult PD patients. The following questions will be addressed: Molecular counterparts of peritoneal semi-permeability, solute and water transport (beyond AQP1), pathomechanisms and molecular and functional impact of peritoneal transformation with low and high GDP fluids, and the respective pathomechanisms and molecular and functional impact of vascular disease in CKD and with different PD fluids. The impact of renal transplantation following PD will be assessed in a subgroup of patients with tenckhoff catheter removal several weeks after transplantation and a functioning graft.