The severity of obstructive sleep apnea is measured by two attended polysomnographies, a baseline PSG performed before and a follow-up PSG performed 6 month after renal transplantation. Overhydration and leg fluid are evaluated by bioimpedance, performed at the beginning and at the end of each polysomnography. Patients who are not already transplanted 6 months after the baseline PSG will be re-assessed and will be analyzed as control group.