Nasal provocation tests (NPTs) with lysine-aspirin (L-ASA) have been recently introduced for assessment of aspirin-induced asthma (AIA). They differ in dose and means of aspirin instillation, duration of observation period, and criteria for positivity. Thus far they have not become a routine part of clinical diagnosis. Fifty-one patients with AIA, confirmed by oral challenge test, were recruited to undergo diagnostic NPTs with L-ASA. In 10 of these patients (19.6%), NPTs could not be performed because of total obstruction of at least one nostril or marked fluctuations in nasal flows, leaving 41 patients with AIA for the study. Control groups consisted of 13 aspirin-tolerant asthmatic patients and 10 healthy subjects. L-ASA at a total dose of 16 mg of acetylsalicylic acid applied bilaterally into the inferior nasal conchae caused significant fall in inspiratory nasal flow in at least one nostril (>40%), which was measured by anterior rhinomanometry, and clinical symptoms of watery discharge and nasal blockage in 35 of 41 patients with AIA, one of 10 healthy subjects, and none of 13 aspirin-tolerant asthmatic patients. No relationship was found between the baseline nasal flow values and the intensity of response to L-ASA. No systemic reactions, including bronchospasm, were noticed, even in patients whose initial FEV1 was lower than 70% of predicted value. This test is highly specific (95.7%) and sensitive (86.7%), but negative results do not exclude possible intolerance to aspirin (predictive value of a negative result 78.6%). In conclusion, the NPT described is a simple, safe, and quick test for diagnosis of AIA. It can be used in patients with unstable asthma. It may be a method of choice to confirm hypersensitivity to aspirin manifested only by symptoms from the upper respiratory tract. Patients suspected of aspirin intolerance who have negative NPT results should undergo bronchial or oral challenge tests with aspirin.