Importance¬ Amyotrophic lateral sclerosis (ALS) has an immune component, but previous human studies have not examined immune changes over time.Objectives¬ To assess peripheral inflammatory markers in participants with ALS and healthy control individuals and to track immune changes in ALS and determine whether these changes correlate with disease progression.Design, Setting, and Participants¬ In this longitudinal cohort study, leukocytes were isolated from peripheral blood samples from 35 controls and 119 participants with ALS at the ALS Clinic of the University of Michigan, Ann Arbor, from June 18, 2014, through May 26, 2016. Follow-up visits occurred every 6 to 12 months. Fifty-one participants with ALS provided samples at multiple points. Immune cell populations were measured and compared between control and ALS groups. Surface marker expression of CD11b+ myeloid cells was also assessed. Changes over time were correlated with disease progression using multivariate regression.Main Outcomes and Measures¬ The number of immune cells per milliliter of blood and the fold expression of cell surface markers. Multivariate regression models were used to correlate changes in immune metrics with changes on the Revised Amyotrophic Lateral Sclerosis Functional Rating Scale (ALSFRS-R).Results¬ Thirty-five controls (17 women 48.6%] and 18 men 51.4%]; mean SD] age, 63.5 9.9] years) and 119 participants with ALS (50 women 42.0%] and 69 men 68.0%]; mean SD] age, 61.4 11.5] years) were enrolled. Compared with controls, participants with ALS had increased mean (SEM) counts (‚Äâ√ó‚Äâ106/mL) of total leukocytes (4.57 0.29; 95% CI, 3.94-5.11] vs 5.53 0.16; 95% CI, 5.21-5.84]), neutrophils (2.87 0.23; 95% CI, 2.40-3.35] vs 3.80 0.12; 95% CI, 3.56-4.04]), CD16+ monocytes (0.03 0.003; 95% CI, 0.02-0.04] vs 0.04 0.002; 95% CI, 0.03-0.04]), CD16‚àí monocytes (0.25 0.02; 95% CI, 0.21-0.30] vs 0.29 0.01; 95% CI, 0.27-0.31]), and natural killer cells (0.13 0.02; 95% CI, 0.10-0.17] vs 0.18 0.01; 95% CI, 0.16-0.21]). We also observed an acute, transient increase in a population of CD11b+ myeloid cells expressing HLA-DR, CD11c, and CX3CR1. Finally, early changes in immune cell numbers had a significant correlation with disease progression measured by change in ALSFRS-R score, particularly neutrophils (‚àí4.37 95% CI, ‚àí6.60 to ‚àí2.14] per 11.47‚Äâ√ó‚Äâ104/mL SD, 58.04‚Äâ√ó‚Äâ104/mL] per year) and CD4 T cells (‚àí30.47 95% CI, ‚àí46.02 to ‚àí14.94] per ‚àí3.72‚Äâ√ó‚Äâ104/mL SD, 26.21‚Äâ√ó‚Äâ104/mL] per year).Conclusions and Relevance¬ Changes in the immune system occur during ALS and may contribute to the pathologic features of ALS.