Abstract Objectives The pulmonary function test (PFT) is a non-invasive and easily available technique to assess respiratory function in patients with amyotrophic lateral sclerosis (ALS); however, patients with dyspnea sometimes show normal PFT findings. Herein, we investigated whether phrenic nerve conduction study (NCS) and PFT are useful to evaluate respiratory function of patients with ALS with normal value ranges in the PFT. Methods We prospectively enrolled 34 patients with definite or probable ALS, who showed FVC (%) ⩾80 of predicted and 78 healthy subjects. PFT and phrenic NCS were performed with the measurement of forced vital capacity (FVC,%), forced expiratory volumes in 1s (FEV1,%), FEV1/FCV ratio (%), and phrenic compound muscle action potential amplitude, and latency. Results Compared to healthy controls, ALS patients showed delayed phrenic nerve latency and the decrease of FVC (%) (p = 0.006 and p < 0.0001, respectively). ROC curve analysis demonstrated that phrenic latency (AUC = 0.7655) and FVC (%) (AUC = 0.8239) discriminated ALS patients from healthy subjects. Conclusion We demonstrated that ALS patients had early respiratory dysfunction, despite normal PFT findings. Significance Phrenic latency and FVC (%) can be helpful to discriminate ALS patients with latent respiratory dysfunction from healthy subjects.