Eighteen amyotrophic lateral sclerosis (ALS) patients having neither respiratory complaints nor subjective symptoms of sleep disturbance were studied by using an ambulatory multi-parameter monitoring system during sleep. They were divided into two groups: 11 patients with predominantly bulbar form and seven with non-bulbar form. After performing daytime pulmonary function tests, the presence of sleep-disordered breathing (SDB) was evaluated by using a portable device. ALS patients did not show significant SDB as a whole, and the respiratory disturbance index (RDI) was not significantly different between the bulbar group and the non-bulbar group. However, three patients of the bulbar group showed significant SDB, and the patterns of apnea/hypopnea suggested that both bulbar weakness and minimal diaphragmatic weakness might cause SDB in ALS patients at an early clinical stage. Multi-parameter respiratory monitoring during sleep should be included in the routine evaluation of ALS patients at an early clinical stage, especially those with predominantly bulbar involvement, in order to predict early respiratory failure.