The detection of respiratory muscle weakness in ALS is necessary to plan initiation of noninvasive positive pressure ventilation and begin discussion of advanced directives. The authors measured the erect seated and supine forced vital capacity (FVC) in 38 patients with ALS and 15 controls. The supine FVC is significantly lower and the erect--supine FVC difference is significantly greater in patients with complaints of dyspnea, orthopnea, and daytime fatigue.