PURPOSE: There is a significant association between insulin resistance and low cardiorespiratory fitness in nondiabetic subjects. In a population with risk factors for type 2 diabetes (T2DM), before they are insulin resistant, we investigated low exercise capacity (VO2max) as an early marker of impaired insulin sensitivity in order to determine earlier interventions to prevent development of insulin resistance syndrome (IRS) and T2DM. METHODS: Cross-sectional analyses of data on 369 (78 men and 291 women) people at risk for IRS and T2DM, aged 45.6 +/- 10 years (20-65 years) old from the Community Diabetes Prevention Project in Minnesota were carried out. The cardiorespiratory fitness (VO2max) by respiratory gas exchange and bicycle ergometer were measured in our at risk non insulin resistant population and compared with a control group living in the same geographic area. Both groups were equally sedentary, matched for age, gender and BMI. RESULTS: The most prevalent abnormality in the study population was markedly low VO2max when compared with general work site screening control group, (n = 177; 137F; 40 M, mean age 40 +/- 11 years; BMI = 27.8 +/- 6.1 kg/m2). Individuals at risk for IRS and T2DM had a VO2max (22 +/- 6 ml/kg/min) 15% lower than the control group VO2max (26 +/- 9 ml/kg/min) (p < 0.001). It was found that VO2max was inversely correlated with HOMA-IR (r = -0.30, p < 0.0001). CONCLUSIONS: Decreased VO2max is correlated with impaired insulin sensitivity and was the most prevalent abnormality in a population at risk for IRS and T2DM but without overt disease. This raises the possibility that decreased VO2 max is among the earliest indicators of IRS and T2DM therefore, an important risk factor for disease progression.