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To examine the role of adipose-resident macrophages in insulin resistance, we examined the gene expression of CD68, a macrophage marker, along with macrophage chemoattractant protein-1 (MCP-1) in human subcutaneous adipose tissue using real-time RT-PCR. Both CD68 and MCP-1 mRNAs were expressed in human adipose tissue, primarily in the stromal vascular fraction. When measured in the adipose tissue from subjects with normal glucose tolerance, covering a wide range of BMI (21-51 kg/m²) and insulin sensitivity (S[sub I]) (0.6-8.0 x 10[sup -4] min[sup -1] ⋅ µU[sup -1] ⋅ ml[sup -1]), CD68 mRNA abundance, which correlated with the number of CD68-positive cells by immunohistochemistry, tended to increase with BMI but was not statistically significant. However, there was a significant inverse relation between CD68 mRNA and S[sub I] (r = -0.55, P = 0.02). In addition, there was a strong positive relationship among adipose tissue CD68 mRNA, tumor necrosis factor-α (TNF-α) secretion in vitro (r = 0.79, P < 0.005), and plasma interleukin-6 (r - 0.67, P < 0.005). To determine whether improving S[sub I] in subjects with impaired glucose tolerance (IGT) was associated with decreased CD68 expression, IGT subjects were treated for 10 weeks with pioglitazone or metformin. Pioglitazone increased S[sub I] by 60% and in the same subjects reduced both CD68 and MCP-1 mRNAs by >50%. Furthermore, pioglitazone resulted in a reduction in the number of CD68-positive cells in adipose tissue and reduced plasma TNF-α. Metformin had no effect on any of these measures. Thus, treatment with pioglitazone reduces expression of CD68 and MCP-1 in adipose tissue, apparently by reducing macrophage numbers, resulting in reduced inflammatory cytokine production and improvement in S[sub I]. Diabetes 54:2305-2313, 2005 [ABSTRACT FROM AUTHOR]