Four patients with myasthenia gravis presented with severe, largely isolated, bulbar and respiratory muscles weakness. Tensilon tests were positive and antiacetylcholine receptor (anti-AChR) antibody titers were negative in all patients. Only 1 patient had a greater than 10% decremental response during the period of respiratory failure. Although routine nerve conduction studies were normal, all had very low-amplitude diaphragmatic compound muscle action potentials. Three patients had abundant fibrillation potentials and positive sharp waves largely restricted to respiratory muscles. Clinical and electrophysiological findings improved with corticosteroids, and surprisingly, decremental responses became positive in all patients. The assessment of patients with largely isolated bulbar and respiratory muscle weakness due to myasthenia gravis may be difficult and misleading, as anti-AChR antibody titers may be negative, decremental responses may be absent, and electrophysiological assessment atypical. Due consideration of clinical symptomatology, a Tensilon test, and a trial of immunosuppression may be necessary to establish the diagnosis.