['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.', 'Sera from patients with FM (n = 47), osteoarthritis (OA) (n = 16), and rheumatoid arthritis (RA) (n = 13) 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 (n = 15), RA (n = 30), systemic lupus erythmatosus (SLE) (n = 30), and systemic sclerosis (SSc) (n = 30) were also analyzed. To determine if seroreactivity correlates with severity, banked sera from patients with FM assessed as severe (n = 28) or mild (n = 37) and from controls (n = 21) were assayed.', 'Following analysis, the prevalence of seroreactivity was found to be higher in patients with FM (22/47, 47%) compared to patients with OA (3/16, 19%; p<0.1) or RA (1/13, 8%; p<0.05) and the autoimmune disease control sera from poly/dermatomyosis (2/15, 13%; p<0.05), and patients with RA (3/30, 10%; p<0.01), SLE (1/30, 3%; p<0.01), and SSc (1/30, 3%; p<0.01). 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%; p<0.05) and controls (4/21, 19%; p<0.01). In addition, both mean threshold and mean tolerance dolorimetry scores were significantly lower in the seropositive patients with mild FM (1.33+/-0.21, 1.95+/-0.25, respectively) compared to the seronegative patients (1.83+/-0.08, 2.53+/-0.11; p<0.05 for both comparisons, respectively).'] Does antipolymer antibody reactivity in a subset of patients with fibromyalgia correlate with severity?