We wished to determine whether the Amyotrophic Lateral Sclerosis Functional Rating Scale-revised (ALSFRS-R) is reliable when used as primary outcome measure in a multicenter clinical trial. To establish inter-rater reliability, we randomly assigned 19 primary raters and 11 back-up raters to score nine amyotrophic lateral sclerosis (ALS) patients using the ALSFRS-R. To assess intra-rater reliability and reliability of telephone administration, we randomly assigned consecutive participants of the Clinical Trial of High Dose Coenzyme Q10 in ALS (QALS) to have in-person ALSFRS-R interviews at both screening and baseline visits (n=41 patients) or to have the ALSFRS-R interview by telephone at screening and in person at the baseline visit (n=27). An intraclass correlation coefficient (ICC) of reliability was calculated using a one-way random effects analysis of variance model. In the inter-rater reliability assessment, the primary raters performed 54 ratings on nine patients with ICC=0.93 (95% CI 0.84-0.98). For back-up raters, 32 ratings on nine patients resulted in ICC=0.93 (95% CI 0.82-0.98). The intra-rater reliability for in-person interviews was ICC = 0.95 (95% CI 0.92-0.98). The reliability of telephone administration compared to in-person interviews was ICC=0.97 (95% CI 0.93-0.98). We conclude that the ALSFRS-R shows excellent inter- and intra-rater reliability, and reliability of telephone administration when used as primary outcome measure in a multicenter ALS trial.