Background: Diaphragm pacing (DP) is an experimental ALS treatment, available through a compassionate use program. Eligibility requires forced vital capacity (FVC) between 45-50% predicted and phrenic nerve conduction study (NCS) evidence showing the diaphragm can be electrically stimulated. Diaphragm ultrasound (DU) also evaluates diaphragm function by demonstrating thickening with inspiration. Methods: A 63 year old man with advanced ALS requested DP as his respiratory functions worsened. He was wheelchair bound and had severe dysarthria and dysphagia. He had exertional dyspnea and used CPAP at night for obstructive apnea. Results: FVC was 47% predicted. Initial phrenic NCS showed a normal response on the right but no response on the left, making him ineligible for DP. Diaphragm function was further assessed with DU. This showed normal thickening with inspiration bilaterally. The DU result prompted repeating the right phrenic NCS which then showed a normal response. He successfully completed surgical implantation of diaphragm leads for DP. At surgery both diaphragms showed good responses to electrical stimulation. Conclusions: Phrenic NCS can be technically challenging and yield a false positive (absent) result. In this patient, DU indicated good diaphragm function, which prompted repeating phrenic NCS. The normal phrenic NCS allowed the patient to pursue DP.