Objective.To determine the prevalence of antipolymer antibodies (APA) in patients with fibromyalgia (FM) and autoimmune disease control groups and to determine if the presence of these antibodies correlates with severity in patients with FM.Methods.Sera from patients with FM, osteoarthritis (OA), and rheumatoid arthritis (RA) were analyzed.Patients with implants of any kind and patients with concurrent autoimmune conditions were excluded from study.Banked sera from autoimmune disease controls including poly/dermatomyosis, RA, systemic lupus erythmatosus (SLE), and systemic sclerosis (SSc) were also analyzed.To determine if seroreactivity correlates with severity, banked sera from patients with FM assessed as severe or mild and from controls were assayed.Results.Following anal., the prevalence of seroreactivity was higher in patients with FM (22/47, 47%) compared to patients with OA (3/16, 19%) or RA (1/13, 8%) and the autoimmune disease control sera from poly/dermatomyosis (2/15, 13%), and patients with RA (3/30, 10%), SLE (1/30, 3%), and SSc (1/30, 3%).The prevalence of APA seroreactivity was also significantly higher in patients with severe FM (17/28, 61%) compared to patients with mild FM (11/37, 30%) and controls (4/21, 19%).In addition, both mean threshold and mean tolerance dolorimetry scores were significantly lower in the seropos. patients with mild FM (1.33, 1.95, resp.) compared to the seroneg. patients (1.83, 2.53; for both comparisons, resp.).Conclusion.These results reveal that an immunol. response, production of antipolymer antibodies, is associated with a subset of patients with FM.The results also suggest that the APA assay may be an objective marker in the diagnosis and assessment of FM and may provide addnl. avenues of investigation into the pathophysiol. processes involved in FM.