Bilateral diaphragmatic paralysis (BDP) is usually caused by anatomic lesions of both phrenic nerves or generalized neurologic diseases. BDP has also been observed during and after infections, associated with mediastinal tumors, or may have an idiopathic etiology. A 57-year-old woman with breast cancer had progressive dyspnea that worsened when in the supine position. Lung function tests and phrenic nerve stimulation revealed bilateral diaphragmatic paralysis. Clinical suspicion of BDP should always be raised in patients suffering from progressive dyspnea and orthopnea. Determination of (VC) when standing and in the supine position and measurement of trandiaphragmatic pressure should reveal this uncommon diagnosis.