For many years it has been suggested that the subjective outcome of the operation of submucosal diathermy to the inferior turbinates (SMD) may be predicted by preoperatively testing the ability of the nasal mucosa to decongest with topical vasoconstrictor agents. This study of 22 patients undergoing SMD, confirms that the pre‐operative decongestion produced by xylometazoline is both a good subjective (P<0.001) and objective (P<0.002) predictor of outcome. If a significant reduction in nasal resistance is achieved by the application of topical decongestants, then the operation of SMD is likely to be effectiv