The role of obstructive sleep apnea (OSA) on chronic kidney disease (CKD) is not clear. This randomized clinical trial will test the impact of OSA treatment on blood pressure (BP) and on the estimated glomerular filtration rate (eGFR) in patients with CKD IIIb and IV (eGFR 44-15 ml/min). A polygraph will be performed to assess the presence of OSA (defined by an apnea-hypopnea index ≥15 events/hour). Patients with OSA will be randomized to use continuous positive upper airway pressure (CPAP) or to maintain optimized clinical treatment for BP control. Antihypertensive medication adjustments will be allowed using a standard protocol for both groups by the same researcher, who will not have access to CPAP follow-up. In addition to clinical (including BP and ambulatory BP monitoring, ABPM) and laboratory assessments at baseline, we will follow up at 3 months, 6 months, 9 months and 12 months after randomization of the proposed outcomes. Target organ damage analyses, such as the retina and echocardiography, will be performed at baseline and after 1 year of randomization. Primary objective: to compare the effect of CPAP on the need to adjust antihypertensive medication to control systolic BP (<130mmHg) in patients with CKD; secondary objectives: 1) to evaluate the reduction in systolic and diastolic BP by office and ABPM; 2) assessment of nocturnal BP dipping; 3) to evaluate the impact of OSA treatment with CPAP on eGFR during follow-up; 4) to evaluate the impact of OSA treatment with CPAP on the evolution of albuminuria; 5) assessment of other target organ damage such as retinopathy and cardiac remodeling; 6) to evaluate the impact of OSA treatment with CPAP on the possible delay for renal replacement therapy or end-stage renal disease (eGFR <15ml/min and dialysis); 7) to evaluate the impact of OSA treatment with CPAP on the quality of life of patients with CKD. With a significance level of 5% and study power of 90%, two-tailed hypothesis testing, 74 patients with OSA per group, i.e., 148 patients in total, will be required to assess the primary endpoint (we estimate that 25% and 50% of patients in control and CPAP groups will not need to adjust their antihypertensive medication at follow-up, respectively).

Inclusion Criteria:

• Confirmed diagnosis of hypertension: Use of oral antihypertensive drugs or systolic BP (SBP) >140 mmHg and/or diastolic BP (DBP) >90 mmHg on 2 office measurements or >130x80 mmHg (24 hours ABPM).
• eGFR by the CKD-EPI equation between 45 ml/min/1.73m2 and 15 ml/min/1.73m2 (in conservative treatment of CKD).

Exclusion Criteria:

• Professional drivers;
• Home supplemental oxygen users;
• Patients under current treatment for OSA;
• Pregnant women;
• Patients with malignant neoplasms or terminal illnesses;
• Severe asthma or chronic obstructive pulmonary disease;
• Patients with systemic lupus erythematosus, amyloidosis or systemic sclerosis;
• Patients with a history of solid organ transplants.