Both the influence of comorbid obstructive sleep apnea-hypopnea syndrome (OSAHS) on daytime sleepiness and its pathological mechanism in 10 narcoleptic patients was studied. Mild elevation of esophageal negative pressure and obstruction in the velopharyngeal area, as observed by upper airway magnetic resonance imaging, during apneic episodes was observed in the subjects. Severity of the disorder was correlated positively with body mass index, and the latencies of the multiple sleep latency tests were not improved with nasal continuous positive airway pressure treatment in subjects. Pathogenesis of OSAHS in narcolepsy was thought to be almost similar to typical OSAHS, and that the disorder seemed to lack a pathological influence on daytime sleepiness.