Whether nasal obstruction disturbs sleep and nocturnal breathing is controversial since convenient techniques for measuring nasal resistance during sleep. Therefore, we developed a technique for unobtrusive, side-selective nasal conductance monitoring. The technique measures left and right nasal airflow and transnasal pressure using nasal cannula, thin catheters inserted through the cannula into the nasopharynx, and 3 pressure transducers. Their processed signals provide conductance as airflow/resistive pressure ratio for the left and right side and the sum, total nasal conductance. For validation, total nasal conductance was also determined by a flowmeter attached to a nasal-mask and nasopharyngeal pressure that served as reference standard. Methods were compared in 5 normals during pharmacological interventions and in 12 snorers during sleep. The novel technique accurately tracked total nasal conductance by the reference method at baseline, after nasal application of histamine and xylomethazoline in normals; mean difference (bias) was 1%, limits of agreement (±2SD of bias) were ±22% (75 comparisons). Corresponding values during overnight sleep studies in snorers were 0%±19% (192 comparisons); bias and limits of agreement of changes in nasal conductance were 1±19% (180 comparisons). Conductance measured once at the beginning of sleep studies differred from subsequent measurements during the night by a mean ±SD of 26 ±20%, p<0.0001. The novel technique accurately measures side-selective conductance. It is suitable to investigate interactions among nasal obstruction, sleep and nocturnal breathing, and drug effects. One-time measurements at the beginning of sleep studies do not appropriately reflect the highly variable nasal conductance during an entire night.