Introduction: The aim of this study was to determine whether a history of pre-morbid type 2 diabetes mellitus (DM2) is a prognostic factor in amyotrophic lateral sclerosis (ALS). Methods: The relationship between DM2 and survival was analyzed in a study population consisting of 1,322 participants from 6 clinical trials. Results: Survival did not differ by diabetes status (log-rank test, P=0.98), but did differ by body mass index (BMI) (log-rank test, P=0.008). In multivariate analysis, there was no significant association between diabetes and survival (P=0.18), but the risk of reaching a survival endpoint decreased by 4% for each unit increase in baseline BMI (HR 0.96, 95% CI 0.94-0.99, P=0.001). DM2 was less prevalent among ALS clinical trial participants than predicted. Conclusions: A history of pre-morbid DM2 is not an independent prognostic factor in ALS clinical trial databases. The low DM2 prevalence rate should be examined in a large, prospective study to determine whether DM2 affects ALS risk.