OBJECTIVE: An unexplained multisymptom illness, Gulf War veterans' illness (GWVI), has been described among allied force veterans of the first Gulf War (1990-1991). It has been proposed that some of its symptoms reflect an immune dysfunction, and rheumatologic symptoms including joint pain and stiffness are reported frequently. However, it is unknown whether synovial inflammation causes the articular symptoms. We examined synovial tissue from individuals with GWVI and joint pain for evidence of inflammation. METHODS: We compared synovial biopsy samples from 6 individuals with GWVI and joint pain with samples from 9 clinically asymptomatic controls (hematoxylin and eosin [H&E] stains only) and biopsy samples or surgically obtained specimens from 10 patients with rheumatoid arthritis (RA) and 12 with osteoarthritis (OA). Inflammatory changes were quantified in H&E stained sections with a modified synovitis score by immunostaining for CD3, CD20, CD38, CD68, Ki-67, and von Willebrand factor, and with a composite inflammation score based on these markers. RESULTS: Normal histology was seen in the GWVI specimens, except for mild focal lining hyperplasia and rare low-grade perivascular infiltrates in 1 specimen each. Mean +/- SEM synovitis scores were lowest and nearly identical in control (1.38 +/- 0.30) and GWVI specimens (1.41 +/- 0.29), intermediate in OA specimens (2.64 +/- 0.39), and highest in RA specimens (6.0 +/- 0.19). Likewise, inflammatory cells, cell division, vascular density, and composite inflammation score were lowest in the GWVI specimens. CONCLUSION: Despite significant joint pain, the GWVI synovia did not differ from normal controls. These results agree with other studies that have failed to document inflammatory or immunologic etiologies in GWVI.