SummaryThe aim of this study was to assess monocyte/macrophage function, as defined by lipopolysaccharide (LPS)-induced production of tumour necrosis factor (TNF)-α, interleukin (IL)-10 and interferon (IFN)-γ by stimulated whole blood cultures in patients with colorectal carcinoma before and after surgical resection. Forty colorectal cancer patients prior to surgery and 31 healthy controls were studied. Heparinized venous blood was taken from colorectal cancer patients prior to surgery and from healthy controls. Serial samples were obtained at least 3-6 weeks post-operatively. Blood was stimulated with LPS for 24 h and supernatants were assayed for TNF-α, IFN-γ and IL-10 by enzyme-linked immunosorbent assay. LPS-induced production of TNF-α and of IFN-γ was reduced in patients with colorectal carcinoma compared to controls (TNF-α, 11 269 pg ml -1 {12 598}; IFN-γ, 0.00 pg ml -1 {226}; median {IQR}) (TNF-α, 20 576 pg ml -1 {11 637}, P < 0.0001; IFN-γ, 1048 {2428}, P = 0.0051, Mann-Whitney U-test). Production in patients after surgery had increased (TNF-α: 17 620 pg ml -1 {7986}; IFN-γ: 410 pg ml -1 {2696}; mean {s.d.}) and were no longer significantly reduced when compared to controls (TNF-α, P = 0.28; IFN-γ, P = 0.76). Production of TNF-α and IFN-γ prior to surgery were reduced to a greater extent in patients with Dukes' stage C tumours compared to those with Dukes' stage A and B stage. There was no difference in IL-10 production between any group. Monocytes/macrophages from patients with colorectal carcinoma are refractory to LPS stimulation as reflected by reduction in TNF-α and IFN-γ production and this is more pronounced in patients with advanced stage tumours. This suppression is not mediated by IL-10 and disappears following surgical resection of the tumour. This provides evidence for tumour induced suppression of immune function in patients with colorectal cancer and identifies a potential therapeutic avenue.