Our objective was to determine the interval from symptom onset to diagnosis, and to evaluate associated factors in a cohort of U.S. Veterans with motor neuron diseases. We retrospectively evaluated 1359 patients enrolled in the National Registry of Veterans with Amyotrophic Lateral Sclerosis (ALS). The main outcome measures were time from symptom onset to first diagnosis and to second opinion. Predictor variables included age at symptom onset, year of symptom onset, race, onset site, final diagnosis, number of diagnostic tests performed and clinical sites visited. Median time to first diagnosis was 11 months; median time to second opinion was two months. In a multivariable model, more recent calendar year of symptom onset, younger age, bulbar onset and a diagnosis of ALS versus non-ALS motor neuron disease were all significantly associated with a shorter time to first diagnosis. Later year of symptom onset and white race were significantly associated with a shorter time to second opinion. While the interval from symptom onset to diagnosis, and many of the associated factors are similar between our large cohort of U.S. Veterans with ALS and other smaller published cohorts, we found that the diagnostic interval among U.S. Veterans has significantly decreased over time.