We aimed at investigating relationship between plasma fibrinogen and insulin sensitivity, which are two major determinants of metabolic Syndrome X (insulin resistance syndrome). We designed a prospective study of 27 non‐diabetic, non‐hypertensive subjects, presenting a wide range of body mass index BMI (10 men, 17 women; mean age
{}\pm{}
SEM: 35.9
{}\pm{}
2.2 years; BMI ranging from 21.1–45.2 kg/m
^{2}
). Insulin sensitivity was assessed with the minimal model procedure, over a 180 min intravenous glucose tolerance test with iterative sampling. Fibrinogen levels were determined by the method of Clauss. The insulin sensitivity index SI (i.e., the slope of the dose–response relationship between insulin increased above baseline and glucose disposal) ranged from 0.0009 to
16 \times 10^{-4}
min
^{-1}
/(
\mu
U/ml), with a mean value of
4.76\pm0.73 \times 10^{-4}
. Mean values of plasma fibrinogen were 3.33
{}\pm{}
0.13 g/l, ranging from 2.21 to 5.07 g/l. There were highly significant negative correlations between SI and the level of plasma fibrinogen (
r= - 0.61
,
p= 0.0007
) and between the basal effect of insulin BIE and plasma fibrinogen (
r = - 0.521
,
p= 0.005
). Basal insulin was positively correlated to fibrinogen (
r = 0.386
,
p = 0.046
). When we analysed the data using partial correlation analysis, the negative relation between SI and fibrinogen was maintained independently from BMI (
r = - 0.45
,
p < 0.05
). These data establish a strong negative association between insulin sensitivity and fibrinogen, involved in the increased cardiovascular risk of metabolic Syndrome X.
