1. TIA with ABCD2 ≥4 or ischemic stroke, within the prior 14 days. 1. pre-event inability to perform all of own basic ADLs 2. unable to obtain informed consent from subject or legally authorized representative 3. incarcerated 4. known pregnancy 5. current mechanical ventilation (can enroll later if this resolves) or tracheostomy 6. current use of positive airway pressure, or use within one month prior to stroke 7. anatomical or dermatologic anomaly that makes use of CPAP interface unfeasible 8. severe bullous lung disease 9. history of prior spontaneous pneumothorax or current pneumothorax 10. hypotension requiring current treatment with pressors (can enroll later if this resolves) 11. other specific medical circumstances that conceivably, in the opinion of the site PI, could render the patient at risk of harm from use of CPAP 12. massive epistaxis or previous history of massive epistaxis 13. cranial surgery or head trauma within the past 6 months, with known or possible CSF leak or pneumocephalus 14. recent hemicraniectomy or suboccipital craniectomy (i.e. those whose bone has not yet been replaced), or any other recent bone removal procedure for relief of intracranial pressure 15. current receipt of oxygen supplementation >4 liters per minute 16. current contact, droplet, respiratory/airborne precautions