The purpose of the study was the assessment of cognitive functioning in 20 children (age 4-14 years) with end stage renal disease (ESRD) before and after kidney transplantations by means of an intelligence-test (WISC-R or WPPSI). Research questions were: I. do the test scores of children with ESRD deviate significantly from those of the norm-group before transplantation; 2. do the test scores of children with ESRD increase significantly from pre- to post-transplantation; 3. do the test scores of children with ESRD after transplantation deviate significantly from those of the norm-group. The results show that children with ESRD before transplantation perform significantly more poorly on the verbal part of the WISC-R. After transplantation they show a significant increase on cognitive functioning, mainly in the verbal part of the test. However, kidney transplantation does not cause a complete recovery of cognitive functioning; after transplantation children with ESRD still showed lower scores on several tasks (e.g. tasks concerning concentration).