Acute respiratory failure can be the product of any of a great number of muscular, neuromuscular, and neurologic causes. The family history may be extremely helpful in narrowing the differential diagnosis. We report the case of a girl who, during the course of a slight upper respiratory infection, presented with acute respiratory failure requiring mechanical ventilation. The family history was significant for a brother who had arthrogryposis and died at 15 h of life, also from respiratory failure. The patient herself had a history of palpebral ptosis in the evening. The absence of electromyographic and muscle biopsy abnormalities and the patient's positive response to anticholinesterase therapy supported the diagnosis of familial infantile myasthenia. We emphasize the importance of considering the myasthenic syndromes in the differential diagnosis of acute respiratory failure, since appropriate therapy can rapidly resolve the symptoms. Furthermore, an accurate diagnosis allows appropriate genetic counseling for the hereditary forms.