With acoustic rhinometry, one can detect a difference in response to a topical decongestant between normal and allergic subjects at the minimal cross-sectional area. The minimal cross-sectional area corresponds in most cases to the anterior end of the inferior turbinate or the first valley, which occurs after the nose piece of the acoustic rhinometer on the acoustic rhinometry graph. Allergic patients typically have an increase in nasal mucosal swelling, which leads to a decrease in the nasal volume and area and, subsequently, to an increase in congestion. In this study acoustic rhinometry was used to test normal and allergic subjects before and after the application of a topical decongestant (1% phenylephrine (Neo-Synephrine)). Symptoms were measured by a five-point scale. Results showed that increasing symptom scores demonstrated a trend toward being related to decreasing area as measured at the minimal cross-sectional area or nasal valve. The average total percent change at the minimal cross-sectional area was calculated for both groups and compared. The normal subjects had an average total percent area change at the minimal cross-sectional area of 15.6% +/- 14.8%, and the allergic subjects had a percent change of 24.6% +/- 20.8%. This represents a significant difference between the normal and allergic subjects in response to the topical decongestant at the minimal cross-sectional area (p = 0.04). However, the average total percent change was not significantly different between the two groups at the second and third valleys. At the second valley, the normal subjects had a percent change of 40% +/- 17.5%, and the allergic subjects had a percent change of 36% +/- 18.5%. At the third valley, the normal subjects had a percent change of 35% +/- 15.4%, and the allergic subjects had a percent change of 32% +/- 20.6% (p = 0.4 and 0.5, respectively). The total area was calculated by adding measurements from both right and left nasal cavities for each subject in an attempt to control the effect of the nasal cycle. Acoustic rhinometry makes it possible to detect a trend in the relationship between either a decrease in area at the minimal cross-sectional area or a decrease in the total volume from 0 to 7 cm into the nasal cavity and congestion. A statistically significant difference between normal and allergic subjects in their response to a topical decongestant at the minimal cross-sectional area was detected by acoustic rhinometry at the nasal valve.