METHODS A 73-year-old man developed progressive respiratory failure within 24 hours, requiring emergency admission for mechanical ventilation. The cause of the dyspnoea and tachypnoea could not be ascertained by routine medical and neurological examination. Neuromuscular disease or a cerebral lesion was considered in the differential diagnosis, together with a cardiovascular cause. METHODS Neither laboratory tests nor additional investigations (chest radiogram, echocardiography) gave a diagnosis. Magnetic resonance imaging and computed tomography excluded a focal cerebral or spinal lesion or a space-occupying lesion in the region of the phrenic nerve. An electrophysiological tests (EPT) failed to establish a neuromuscular disease. However, when signs of upper motor neurone irritation appeared, more detailed EPTs, also of the muscles of breathing, provided the diagnosis of amyotrophic lateral sclerosis (ALS). METHODS Symptomatic treatment with pyridostigmine made it possible gradually to wean the patients off the ventilator so that he could be sent home on intermittent mechanical ventilation. CONCLUSIONS In case of acute respiratory failure without cardiopulmonary cause motoneurone disease with initial involvement of respiratory musculature should be considered, even in the absence of clinical signs, and special electrophysiological tests may be necessary to recognize the underlying disease.