Reduced HLA-DR expression on monocytes has been suggested as a predictive marker of immunosuppression following very high risk surgery, but there are few reports in lower risk surgery. In 32 patients undergoing low to intermediate risk surgery, blood samples were analysed by flow cytometry for HLA-DR expression and numbers in both CD14(high) and CD14(low)CD16+ monocyte subsets. The numbers of CD14(high) monocytes increased at 24 h (mean (SD), 5.0 (2.2) vs 7.6 (3.9) x 10(5) cells.ml(-1); p < 0.01) while CD14(low)CD16+ monocytes decreased (0.68 (0.36) vs 0.44 (0.36) x 10(5) cells.ml(-1); p < 0.01). HLA-DR expression was significantly reduced in both subsets by 24 h (mean (SD) fluorescent intensity 440 (310) vs 160 (130) for CD14(high) and 1000 (410) vs 560 (380) for CD14(low)CD16+ subsets; p < 0.01). This reduction of monocyte HLA-DR expression 24 h following lower risk surgery raises questions about the purported clinical utility of this biomarker as an early predictor of postoperative complications. Our results also suggest that surgery induces significant trafficking (i.e. mobilisation, margination and extravasation) of monocyte subsets, and that monocyte HLA-DR depression is the result of a down-regulatory phenomenon (decreased protein expression on each cell) rather than the differential trafficking of monocyte subsets.