Clinical investigations examining biomarker candidates in Gulf War Veterans' Illnesses (GWVI) are limited, and studies that encompass both baseline and 10‐year follow‐up data for blood biomarker candidates are currently unavailable. We thus propose to conduct a 10‐year follow‐up investigation in Gulf War Veterans to examine promising blood biomarker candidates at baseline and 10‐year follow‐up, a project that has the potential to be rapidly catalyzed into clinically efficacious interventions for GWVI. Our preliminary baseline data in serum samples collected 2005 ‐ 2014 from Veterans who served in the Gulf War era (1990‐1991) suggest significant differences in a number of biomarker candidates in Veterans who had been deployed to the Gulf War theater (compared to Veterans who had served in the 1990‐1991 Gulf War era, but who had not been deployed to the region of conflict). Baseline pilot studies suggest that Veterans with a history of deployment to the Gulf War theater demonstrate: a.) altered neurosteroid metabolism, possibly secondary to endocrine disruption involving chemical agent exposures (among other potential etiologies), b.) reductions in Vitamin D (implicated in processes relevant to GWVI, including inflammation, cancer, and environmental exposures, c.) elevations in acylcarnitines and other endogenous esters (potentially signaling mitochondrial changes), and d.) elevations in c‐reactive protein (a marker of inflammation likely relevant to GWVI). Our recent data demonstrate that many of these analytes are also significantly associated with possible symptoms of GWVI, further strengthening the rationale for this project. All of these biomarker candidates have the potential to be rapidly translated into new treatments for GWVI, consistent with Institute of Medicine's (IOM) recent recommendations that “Future Gulf War research should place top priority on the identification and development of effective therapeutic interventions and management strategies for Gulf War illness” (2016). We thus propose to examine biomarker candidates at baseline and also at 10‐year follow‐up in Veterans of the 1990‐1991 Gulf War era to inform therapeutic development. 1. Based on strong pilot data, we propose to conduct baseline biomarker investigations in 527 Gulf War era Veterans for whom serum samples are already collected. We hypothesize that we will replicate our pilot findings showing alterations in neurosteroid metabolism, Vitamin D, acylcarnitines, and inflammatory markers in Gulf War Veterans with a history of deployment to the Gulf War theater of operations. 2. As 94% of the 527 Gulf War era Veterans who provided a blood sample between 2005 – 2014 have already provided explicit permission to be re‐contacted for future research studies (i.e. agreeing to this in the consent form of the original 2005 – 2014 study), and as our recent feasibility study shows that 75% of previously enrolled Veterans can indeed be successfully r...