The purpose of this study was to determine the effect of acute nasal obstruction on sleep and breathing in eight normal persons. The subjects were randomized into two groups. One night the subject was studied with the nose open and a second night with the nose obstructed. The electroencephalogram, electrocardiogram, inspiratory effort, nasal and oral airflow, and oxygen saturation were monitored. Sleep proved to be both subjectively and objectively disturbed. The subjects with the nose obstructed awoke more often, had a greater number of changes in sleep stage, had a prolongation of rapid-eye-movement (REM) latency, and spent a greater amount of time in stage I non-REM sleep (light sleep). Acute nasal obstruction caused a statistically significant increase in the number of partial and total obstructive respiratory events (obstructive hypopnea and obstructive apnea). Sleep apnea developed in one subject during this study merely on the basis of acute nasal obstruction.