Nasal prong pressure monitoring (PNOSE) is utilized to assess ventilation during sleep. However, it has not been rigorously validated against the gold standard of face-mask pneumotachography (VFM). Therefore, we compared PNOSE with VFM in 20 patients with suspected sleep apnea during nocturnal polysomnography, and analyzed factors affecting accuracy of PNOSE-derived variables. Patients rated their nasal obstruction on a visual analog scale. Mean +/- SE apnea/hypopnea index (AHI) by VFM was 24.0 +/- 5.1 h(-1). The bias (mean difference) and limits of agreement (+/- 2 SD) of AHI derived from PNOSE, and square root-transformed PNOSE, a measure proposed as a surrogate of airflow, were +3.9 (+/- 4.6), and -0.9 (+/- 9.0) h(-1). Subjective scores of nasal obstruction before polysomnographies did not herald inaccuracy of AHI from PNOSE. Square root-transformed PNOSE closely tracked pneumotachographic airflow over 10 breaths (r(2) among signals 0.88 to 0.96) but the relationship among these signals was highly variable if comparisons were extended over an entire night. Compared with face-mask pneumotachography, nasal pressure monitoring provides accurate AHI for clinical purposes even in patients perceiving nasal obstruction. Square-root transformation provides near linear nasal pressure/airflow relationships over a short time but is not essential for estimation of AHI.