Background: Guillain-Barré syndrome (GBS) is an acute acquired autoimmune inflammatory disorder of the peripheral nerves and roots. Respiratory insufficiency is an important predictor of a poor prognosis in patients with GBS. Phrenic nerve assessment is an area of interest in GBS with respiratory failure. We aimed to analyze the characteristics of the phrenic nerve conduction study (NCS) in GBS and assess its value in predicting respiratory failure requiring ventilatory support, along with outcome assessment at 6 months as per the Hughes score. Methods: A total of 135 patients with GBS admitted to our hospital over 3 years were thoroughly evaluated by clinical examination, blood laboratory tests, and phrenic NCS. Results: Phrenic NCS abnormality was observed in 48 patients (35.6%) with statistically significant increases in phrenic sum compound muscle action potential (CMAP) latency (18.91±7.82 ms) and sum CMAP duration (44.65±6.84 ms), along with reduced sum CMAP amplitude (0.3246±0.132 mV) and sum CMAP area (3.56±2.62 mV·ms) occurring in those requiring ventilatory assistance. The sensitivity, specificity, positive and negative predictive values, and positive and negative likelihood ratios of the phrenic NCS for predicting respiratory failure in patients with GBS were 90.7%, 90.2%, 81.3%, 95.4%, 9.27, and 0.10, respectively. The results showed a statistically significant association between abnormal initial phrenic NCS scores and outcomes at 6 months. Conclusion Phrenic NCS can predict respiratory failure requiring ventilator assistance in patients with GBS. Phrenic NCS can be incorporated into routine NCS protocols to predict impending respiratory failure in patients with GBS.