Among 250 laboratory-born bonnet macaques living in social groups and maintained on commercial monkey chow, we measured weight, crown-rump length, sagittal abdominal diameter (SAD), and fasting serum insulin, glucose, triglycerides, high-density lipoproteins, low-density lipoproteins, and total cholesterol. Body mass index (BMI = weight/crown-rump length(2)), and insulin resistance determined by the insulin/glucose ratio (IGR) and homeostasis model assessment, were measured. We defined the metabolic syndrome using a composite score based on morphometry, insulin resistance, and serum lipid levels, analogous to clinical criteria. Elevated BMI was associated with significantly greater SAD, insulin, IGR, homeostasis model assessment, and triglycerides. Among 120 adult monkeys aged 5-17 yr, males (n = 48) had higher BMI, SAD, insulin, and IGR levels than females, independent of age. Sixteen of 113 adult monkeys and five of 36 peripubertal subjects, aged 3-4 yr (14%), met our criteria for the metabolic syndrome, as did four of 12 monkeys, aged 20-28 yr. Markers of the metabolic syndrome are present by 3-4 yr of age in our colony and are observed across the life span in the absence of conventional obesifying interventions. Socially reared and housed bonnet macaques may provide a useful model for studying the pathogenesis, prevention, and treatment of the metabolic syndrome.