Background: Neuromuscular junction transmission impairment has been described in amyotrophic lateral sclerosis (ALS). The degree of repetitive nerve stimulation (RNS) decrement may correlate with disease activity if measurement is repeatable. We determined test-retest correlation of decremental response in patients with ALS. Methods: RNS (3 Hz) was assessed by blinded technician and clinician on median - abductor pollicis brevis (median-APB), ulnar - adductor digiti minimi (ulnar-ADM), and/or accessory - trapezius (accessory-TRAP) nerve-muscle pairs during two evaluations. Repeatability was assessed by measuring the strength of test-retest correlation using the Pearson correlation coefficient (r). Results: 24 patients were included. Decrement was measured for 16 patients on median-APB, 22 on ulnar-ADM, and 24 on accessory-TRAP. Repeated measures of decrement demonstrated strong test-retest correlation for median-APB (r = 0.82, p < 0.001) and accessory-TRAP (r = 0.87, p < 0.001). Correlation was poor in ulnar-ADM (r = −0.01, p = 0.949). Correlation was superior in median-APB muscles with higher Medical Research Council (MRC) grade strength (MRC ≥ 4, r = 0.89, p = 0.001 vs MRC < 4, r = 0.73, p = 0.062). Conclusions: Decremental response in patients with ALS has strong test-retest correlation in median-APB and accessory-TRAP, but not in ulnar-ADM.