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Objective: In this study, we evaluated the modifications of nasal function in type 1 diabetes (insulin-dependent diabetes mellitus [IDDM]) by active anterior rhinomanometry (AAR) to understand if involvement of the nasal nervous system and microcirculation could be detected in nasal mucosa., Method: We studied 35 nonsmoking IDDM patients without diabetic complications, nasal pathology, or septal deviation. We measured serum levels of nitric oxide (NO) and nasal airway in three conditions: basal, supine, and after decongestion (phenylephrine hydrochloride 0.25 mg spray) by means of rhinomanometry, determining inspiratory total resistance and nasal airflow. The rhinomanometric results of the IDDM patients were compared with those of control normal subjects. In the IDDM patients, neuropathy was evaluated according to standardized procedures, including the vibration perception threshold test, cardiovascular autonomic tests, conduction velocity test, and fundoscopic examination., Results: The NO serum level was significantly higher in IDDM patients (12.5 +/- 3.8) compared with normal controls (4.8 +/- 1.4). The AAR results showed that in IDDM patients, inspiratory total resistance in the basal (0.82 +/- 0.4 Pa/cm3) and supine (0.94 +/- 0.7 Pa/cm3) positions and after decongestion (0.59 +/- 0.2 Pa/cm3) were increased compared with the control group in three conditions (basal, 0.52 +/- 0.2 Pa/cm3; supine, 0.58 +/- 0.3 Pa/cm3; after decongestion, 0.48 +/- 0.2 Pa/cm3). After decongestion, there was a greater decrease in nasal resistance in diabetic patients than in normal subjects., Conclusion: Nasal function is involved in IDDM, rhinomanometry can also be considered an important test in the evaluation of this involvement in patients without other signs of diabetic neuropathy, and an increase in NO could partially explain these alterations.