Background and Purpose-: Obstructive sleep apnea (OSA) is common after acute ischemic stroke and predicts poor stroke recovery, but whether screening for OSA and treatment by continuous positive airway pressure (CPAP) improves neurological outcome is unknown. We used a cost-effectiveness model to estimate the magnitude of benefit that would be necessary to make polysomnography (PSG) and OSA treatment cost-effective in stroke patients. Methods-: A decision tree modeled 2 alternative strategies: PSG followed by 3 months of CPAP for those found to have OSA versus no screening. The primary outcome was the utility gained through OSA screening and treatment in relation to 2 common willingness-to-pay thresholds of $50 000 and $100 000 per quality-adjusted life year (QALY). Results-: Screening resulted in an incremental cost-effectiveness ratio of $49 421 per QALY. Screening is cost-effective as long as the treatment of stroke patients with OSA by CPAP improves patient utilities by >0.2 for a willingness-to-pay of $50 000 per QALY and 0.1 for a willingness-to-pay of $100 000 per QALY. Conclusions-: A clinical trial assessing the effectiveness of CPAP in improving stroke outcome is warranted from a cost-effectiveness standpoint. (C) 2005 American Heart Association, Inc.