The diagnostic criterion for amyotrophic lateral sclerosis (ALS) based on the findings of concomitant clinical and electrophysiological evidence of upper and lower motor neuron involvement may remain unsatisfied for months and in some patients, even for years in the early stage of the disease. Since respiratory involvement is an onset symptom of ALS in only 1-3% of patients, pulmonary assessment has never been considered useful in the early diagnosis of ALS. However, studies on pulmonary function are lacking, especially in those early stages where neurologic tests are also inconclusive. In contrast to the scarcity of data in the early stages, as the disease progresses, it is increasingly enriched by a rich set of symptoms and positive respiratory tests until respiratory failure occurs, which represents the main cause of death in ALS. Hereby we analyze the main pulmonary function tests (PFT) in the various stages of the disease, up to the recent evidence for the possibility of an early diagnosis.