 Obstructive sleep apnea OSA and associated hypoxemia during sleep have been associated with a decline in kidney function. The abnormalities in kidney function associated with OSA have been improved by treatment of OSA with continuous positive airway pressure CPAP, which is a common and effective therapy for OSA. Up to now, clinical research on the impact of OSA on kidney function has been performed on patients without CKD. This study will examine the impact of CPAP therapy on kidney function in patients with CKD. 