Adipose tissue expresses tumor necrosis factor (TNF) and interleukin (IL)-6, which may cause obesity-related insulin resistance. We measured TNF and IL-6 expression in the adipose tissue of 50 lean and obese subjects without diabetes. Insulin sensitivity (S(I)) was determined by an intravenous glucose tolerance test with minimal-model analysis. When lean [body mass index (BMI) <25 kg/m(2)] and obese (BMI 30-40 kg/m(2)) subjects were compared, there was a 7.5-fold increase in TNF secretion (P < 0.05) from adipose tissue, and the TNF secretion was inversely related to S(I) (r = -0.42, P < 0.02). IL-6 was abundantly expressed by adipose tissue. In contrast to TNF, plasma (rather than adipose) IL-6 demonstrated the strongest relationship with obesity and insulin resistance. Plasma IL-6 was significantly higher in obese subjects and demonstrated a highly significant inverse relationship with S(I) (r = -0.71, P < 0.001). To separate the effects of BMI from S(I), subjects who were discordant for S(I) were matched for BMI, age, and gender. By use of this approach, subjects with low S(I) demonstrated a 3.0-fold increased level of TNF secretion from adipose tissue and a 2.3-fold higher plasma IL-6 level (P < 0.05) compared with matched subjects with a high S(I). Plasma IL-6 was significantly associated with plasma nonesterified fatty acid levels (r = 0.49, P < 0.002). Thus the local expression of TNF and plasma IL-6 are higher in subjects with obesity-related insulin resistance.