Decreased heart rate variability is an independent risk factor for cardiac mortality in hemodialysis patients. Our aim was to determine whether it is already present in uremic children and young adults on hemodialysis and following renal transplantation. Twenty-two hemodialysis patients [age 17.2 years (median, quartiles 13.0-22.6)], 22 transplant patients [18.4 years (14.4-21.2)], and 29 healthy controls [16.4 years (15.7-21.1)] were examined. Heart rate and its high (HF) and low (LF) frequency variability were measured in the supine position for 10 min. High and low frequency variability was significantly reduced, whereas heart rate and LF/HF ratio was significantly elevated in both patient groups compared with controls. There was a clear-cut difference between the dialyzed and the transplanted groups based on the HF variability, with the lowest values in the dialysis group ( P<0.01). LF and LF/HF data did not allow us to distinguish between the patient groups. In conclusion, heart rate variability in the HF range is a sensitive tool for detecting cardiovascular autonomic dysfunction that is already present in children and adolescents with impaired kidney function.