OBJECTIVE: It remains unknown whether sex impacts on intramyocellular triglyceride (IMTG) in obesity, as has been shown in non-obese subjects, and, if so, whether this may have implications on the association between IMTG and insulin sensitivity. SUBJECT AND METHODS: A muscle biopsy from vastus lateralis was obtained in 27 obese women (body mass index (BMI)=35.5+/-0.8 kg/m(2); mean+/-s.e.m., percentage of body fat (PBF)=44+/-1, n=7 impaired fasting glucose, n=7 type 2 diabetes), 20 obese men (BMI=35.8+/-0.8 kg/m(2); PBF=33+/-1, n=4 impaired-fasting-glucose; n=6 type 2 diabetes) and 12 lean sedentary healthy individuals (controls; n=7 women, BMI=21.8+/-0.7 kg/m(2), PBF=20+/-2; n=5 men, BMI=23.6+/-0.5 kg/m(2), PBF=13+/-2). IMTG was determined by chromatography. RESULTS: IMTG was increased twofold in obese women compared to obese men, lean men and lean women respectively (21.9+/-2.4 mg/g wet weight, 10.9+/-1.5, 9.8+/-2.1 and 10.9+/-2.4 mg/g, P<0.001). Among obese subjects of either gender IMTG did not increase along with reduced glycaemic control in terms of impaired fasting glucose and diabetes. Plasma insulin levels, which were similar among obese women with different glycaemic control levels, but much lower in lean women, paralleled the changes in IMTG among women. PBF was associated with IMTG in all subjects (P<0.001). In a linear model, sex (P<0.05) and PBF (P<0.05) independently explained variation in IMTG. Plasma free fatty acids (FFA) correlated with IMTG in all subjects (P<0.005). CONCLUSION: Obese women display twice as much IMTG as obese men matched for BMI. Increased IMTG could be a pathophysiological element or a mere physiological phenomenon in feminine obesity ensuing prior to impaired glycaemic control, but associated with increased body fat, circulating FFA and insulin.