In 30 normal subjects and 20 patients complaining of snoring or sleep apnea nasal percentage of total respiratory air volume (nasal fraction) was measured when nasal resistance was gradually increased. The switching point from nasal to oral breathing was also examined. Nasal respiratory resistance was measured by a rhinograph, and nasal fraction was measured with an inductive body-plethysmograph and with a CPAP nasal mask when the patients were at rest. Several sizes of cotton pledgets were inserted into the nasal vestibules as obturators, and nasal respiratory resistance and nasal fraction were measured repeatedly until subjects had to breathe through the mouth completely. Although the value of nasal respiratory resistance in patients with complaints of snoring or sleep apnea was higher than that of normal subjects, most of the patients did not complain of a sensation of nasal obstruction. However, the switching point from nasal to oro-nasal breathing in patients with complaints of snoring or sleep apnea was statistically higher than that in normal subjects. These results suggest that patients with complaints of snoring or sleep apnea can easily breathe through the mouth during sleep, and that chronic nasal obstruction may induce obstructive sleep apnea. Therefore, measurements of nasal respiratory resistance and nasal fraction are useful to evaluate obstructive sleep apnea.