The potential role of anti-inflammatory cytokines in human obesity is unknown. We tested the hypothesis that low serum IL-10 concentrations associate with the metabolic syndrome in obese women. Compared with 50 matched nonobese women, the prevalence of the metabolic syndrome (>/=3 of the following abnormalities: waist circumference, >88 cm; triglycerides, >1.69 mmol/liter; high density lipoprotein cholesterol, <1.29 mmol/liter; blood pressure, >130/85 mm Hg; glucose, >6.1 mmol/liter) was higher in 50 obese women (52% vs. 16%; P < 0.01). As a group, obese women had higher circulating levels of IL-6, C-reactive protein, and IL-10 than nonobese women. In both obese and nonobese women, IL-10 levels were lower in those with than in women without the metabolic syndrome: obese, 1.3 (0.7/2.1) pg/ml vs. 4.5 (4.3/7.4) pg/ml (median and quartiles; P < 0.01); and nonobese, 0.9 (0.7/1.3) pg/ml vs. 1.3 (0.9/3.3) pg/ml (P < 0.05). After 12 months of a lifestyle program, body weight decreased by 10.9 +/- 1.7 kg and was associated with a significant decrement of IL-6, C-reactive protein, and IL-10 levels; the decrease in IL-10 levels was confined to obese women without the metabolic syndrome. These results show that circulating levels of the anti-inflammatory cytokine IL-10 are elevated in obese women and that low IL-10 levels are associated with the metabolic syndrome.