BACKGROUND: Patients treated with nasal continuous positive airway pressure (nasal CPAP) for obstructive sleep apnea (OSA) often have adverse effects from and poor adherence to CPAP. OBJECTIVE: To describe abnormal CPAP leak profiles and assess inter-observer reliability in identifying leak profiles and the correlation of leak profiles with leak rate and clinical outcomes. METHODS: In a sleep-disorders clinic we prospectively studied 35 consecutive patients newly diagnosed with moderate or severe OSA, and who had undergone polysomnographic diagnosis and nasal CPAP titration. We analyzed the data recorded by their CPAP machines during their first week of CPAP. Two independent clinical sleep specialists inspected each night's leak profiles. We defined a “continuous” leak profile segment as a leak increase of ≥ 20 L/min for > 5 min. We defined a “serrated” leak profile segment as a leak that oscillated up to ≥ 20 L/min in ≤ 5 min. With a validated questionnaire, we surveyed the patients about adverse effects. RESULTS: Overall inter-observer agreement was 88% for continuous leak and 92% for serrated leaks. The kappa values were 0.76 and 0.85, respectively. Deviance (± 2 SD) between scorers was −14% to 11% for continuous leaks, and −15% to 9% for serrated leaks. The duration of manually scored profiles correlated modestly but significantly with the machine-recorded leaks. The mean ± SD adherence to CPAP was lower in the patients with the highest quartile of continuous leak (5.28 ± 2.24 h/night versus 6.66 ± 1.72 h/night). Adverse effects increased with increasing serrated leak ( P = .01). CONCLUSIONS: Manually scored leak profiles in patients treated with nasal CPAP can guide clinicians with respect to short-term adherence to nasal CPAP and adverse effects.