Forced vital capacity (FVC) and slow vital capacity (SVC) are non-invasive tests of respiratory function. Although FVC has been extensively studied and is used in most PFT (pulmonary function test) labs, SVC can also be used in clinical practice as it is a more comfortable and convenient test to perform. SVC-based diagnostic criteria can lead to earlier detection of obstructive lung disease. In contrast to FVC, SVC is less affected by respiratory muscle fatigue, airflow patency, expiratory muscle weakness and air leakage making it an appropriate test of respiratory function in patients of amyotrophic lateral sclerosis (ALS) and other neuromuscular disorders. As respiratory insufficiency is the major cause of mortality in ALS patients, regular SVC measurement provides the respiratory functional status, so that early treatment can be started which improves the survival and quality of life in these patients. The purpose of this article is to highlight the importance of considering SVC in clinical practice.