Impairment of pulmonary function is a major prognostic indicator in amyotrophic lateral sclerosis (ALS). Forced vital capacity (FVC) and maximal voluntary ventilation (MVV) decline linearly and are commonly used to assess disease progression. The aim of this study was to evaluate the usefulness of testing respiratory muscle strength in ALS with a novel test, sniff nasal pressure (Pn(sn)), in parallel with more classic tests such as maximal inspiratory pressure (PI(max)) and maximal expiratory pressure (PE(max)). Sixteen patients with ALS were examined monthly over a period of 18 +/- 10 months. At the time of inclusion in the study, values were normal for FVC (107% of predicted value) and MVV (87% of predicted value) but abnormally low for Pn(sn) (67% of predicted value), PI(max) (69% of predicted value), and PE(max) (54% of predicted value). Late in the course of ALS, all patients could perform Pn(sn) whereas 6 could not perform PI(max) and 7 could not perform PE(max). The rate of deterioration was most often linear and similar for FVC (-4.1% of predicted value per month), MVV (-4.3% of predicted value per month), and Pn(sn) (-4.2% of predicted value per month). We conclude that Pn(sn) was the single respiratory test combining linear decline, sensitivity in mild disease, and feasibility in advanced disease. Being easy to perform and inexpensive, Pn(sn) appears well suited to assess the decline of respiratory muscle strength in ALS.