Recent muscle biopsy studies have shown a relation between intramuscular lipid content and insulin resistance. The aim of this study was to test this relation in humans by using a novel proton nuclear magnetic resonance (1H NMR) spectroscopy technique, which enables non-invasive and rapid (approximately 45 min) determination of intramyocellular lipid (IMCL) content. Normal weight non-diabetic adults (n = 23, age 29+/-2 years. BMI = 24.1+/-0.5 kg/m2) were studied using cross-sectional analysis. Insulin sensitivity was assessed by a 2-h hyperinsulinaemic (approximately 450 pmol/l)-euglycaemic (approximately 5 mmol/l) clamp test. Intramyocellular lipid concentrations were determined by using localized 1H NMR spectroscopy of soleus muscle. Simple linear regression analysis showed an inverse correlation (r = -0.579, p = 0.0037) [corrected] between intramyocellular lipid content and M-value (100-120 min of clamp) as well as between fasting plasma non-esterified fatty acid concentration and M-value (r = -0.54, p = 0.0267). Intramyocellular lipid content was not related to BMI, age and fasting plasma concentrations of triglycerides, non-esterified fatty acids, glucose or insulin. These results show that intramyocellular lipid concentration, as assessed non invasively by localized 1H NMR spectroscopy, is a good indicator of whole body insulin sensitivity in non-diabetic, non-obese humans.