Increased concentrations of plasma fibrinogen, an independent risk factor for cardiovascular disease (CVD), in obese children have been reported. The underlying mechanism for this, however, remains to be defined. In the current study, we measured the fractional synthesis rates (FSR) of plasma fibrinogen in six healthy postpubertal obese girls [body mass index (BMI) 36.6 ± 1.8 kg/m²; age 16.6 ± 0.5 yr] and six age-matched lean normal control girls (BMI 20.8 ± 0.7 kg/m²; age 16.4 ± 0.4 yr) during a primed, continuous infusion of L-[1-¹³C]leucine in the postabsorptive state. The method involved purification of plasma fibrinogen by use of immunoaffinity chromatography followed by measurement of [¹³C]leucine enrichment using gas chromatography-combustion-isotope ratio mass spectrometry. The FSR of fibrinogen in obese girls (35.06 ± 2.61%/day) was almost double that in lean girls (17.02 ± 1.43%/day), and this increase was associated with a relative increase in plasma concentration of fibrinogen as well as BMI in the subjects studied. Obese subjects had high fasting insulin levels (138 ± 47 pmol/l) compared with lean subjects (54 ± 11 pmol/l), whereas their glucose concentrations were similar (4.5 ± 0.3 mmol/l in obese and 4.4 ± 0.4 mmol/l in lean subjects), suggesting insulin resistance. The doubling of the FSR of fibrinogen provides novel insight into the mechanism of elevated levels of plasma fibrinogen and suggests a primary role for increased synthesis in producing the hyperfibrinogenemia associated with obesity. This finding may have important implications in the design of therapies for modulating plasma fibrinogen levels in obesity and/or CVD in childhood.