The alternating congestion and decongestion of the nasal mucosa, which is called the nasal cycle, is well documented by several investigators. Most of the previous studies of the nasal airway patency have been accomplished with rhinomanometry. But in the recent reports, acoustic rhinometry is a useful diagnostic tool, which is a simple, accurate and quantitative method for study of nasal airway patency. We assessed the change of the geometry of the nasal cavity with nasal cycle over a 12-hour period in 20 normal adults by acoustic rhinometry. Sixteen subjects(80%) had at least one nasal cycle during observation period, which had a reciprocal change between two sides of nasal cavity during the nasal cycle. The duration of a cycle was variable person to person, ranging from 100 minutes to 380 minutes, but many subjects were distributed around 200 minutes. The degree of cyclic variation in cross-sectional area of C-notch and nasal cavity volume were 39.5% and 45.3%, respectively. Cross-sectional area and nasal cavity volume in decongested phase were a little less than those after decongestion with phenylephrine spray, but the differences were not significant statistically.