The sternocleidomastoid muscle (SCM) is an accessory inspiratory muscle, but it is not investigated systematically in patients with amyotrophic lateral sclerosis (ALS). We aimed to study the involvement of the SCM and to evaluate the role of the diaphragm and SCM on respiratory tests performed in ALS patients. We studied 45 patients (mean age +/- SD: 60.6 +/- 13 years). In all patients we evaluated: neck flexion strength; forced vital capacity (FVC); maximal inspiratory pressure (PImax); sniff nasal inspiratory pressure (SNIP); ALS functional scale (ALS-FRS-R); SCM and diaphragm compound muscle action potential (CMAP) amplitudes (SCM Ampl and Diaphr Ampl) and latencies (SCM Lat and Diaphr Lat). In ALS patients, SCM Lat is increased and SCM Ampl is smaller in patients with neck weakness. The subgroup of patients with neck weakness had more abnormal respiratory function tests and lower clinical scores. There is a significant correlation between SCM Amp and Diaphr Ampl, FVC, PImax, and SNIP. Hence, there is a parallel loss of motor units in the SCM and diaphragm. On multiple regression analysis both PImax and SNIP are dependent on SCM Ampl and Diaphr Ampl, but FVC is not. PImax and SNIP determination mostly depend on SCM and diaphragm function, but the FVC is also dependent on expiratory muscle function. We conclude that neck weakness is a clinical sign that indicates a poor prognosis, and the SCM CMAP can contribute to respiratory function evaluation in ALS patients.