Background/Aims Abnormal immune regulation and increased intestinal permeability augmenting the passage of bacterial molecules that can activate immune cells, such as monocytes/macrophages, have been reported in irritable bowel syndrome (IBS). The aim was to compare the maturation phenotype of monocytes/macrophages (CD14+) from IBS patients and controls in the presence or absence of Escherichia coli lipopolysaccharides (LPS), in vitro. Methods Mononuclear cells were isolated from peripheral blood of 20 Rome II-IBS patients and 19 controls and cultured with or without LPS for 72 hours. The maturation phenotype was examined by flow cytometry as follows: M1-Early (CD11c(+)CD206(-)), M2-Advanced (CD11c(-)CD206(+)CX3CR1(+)); expression of membrane markers was reported as mean fluorescence intensity (MFI). The Mann-Whitney test was used and significance was set at P < 0.05. Results In CD14+ cells, CD11c expression decreased with vs without LPS both in IBS (MFI: 8766.0 +/- 730.2 vs 12 920.0 +/- 949.2, P < 0.001) and controls (8233.0 +/- 613.9 vs 13 750.0 +/- 743.3, P < 0.001). M1-Early cells without LPS, showed lower CD11c expression in IBS than controls (MFI: 11 540.0 +/- 537.5 vs 13 860.0 +/- 893.7, P = 0.040), while both groups showed less CD11c in response to LPS (P < 0.01). Furthermore, the percentage of "Intermediate" (CD11c(+)CD206(+)CX3CR1(+)) cells without LPS, was higher in IBS than controls (IBS = 9.5 +/- 1.5% vs C = 4.9 +/- 1.4%, P < 0.001). Finally, fractalkine receptor (CX3CR1) expression on M2-Advanced cells was increased when treated with LPS in controls but not in IBS (P < 0.001). Conclusions The initial phase of monocyte/macrophage maturation appears to be more advanced in IBS compared to controls. However, the decreased CX3CR1 in patients with IBS, compared to controls, when stimulated with LPS suggests a state of immune activation in IBS.