Background As immune regulatory and effector cells, monocytes play an important role in the blood-extracorporeal circuit contact-related acute lung injury in patients undergoing cardiopulmonary bypass (CPB). However, circulating monocytes are phenotypically and functionally heterogeneous, so we characterised how immature monocytes affect acute lung injury induced by CPB. Methods The identification and dynamic changes in monocyte subsets were monitored by flow cytometry in patients undergoing CPB and in a rat model of CPB. The differentiation and migration of monocyte subsets were explored by in vitro cultures and adoptive transfer in the CPB rat model. Results We observed a dramatic increase of two monocyte subsets in the peripheral blood of patients undergoing CPB, involving tumour necrosis factor (TNF) alpha- producing, mature intermediate CD14(high)CD16(+) monocytes and a novel immature CD14(low)CD16-subset. The immature CD14(low)CD16-monocytes possessed limited ability for TNF-alpha production, and failed to suppress T-cell proliferation mediated by T-cell receptor signalling. However, these immature cells were highly proliferative and could differentiate into TNF-alpha producing, mature CD14(high)CD16+ monocytes. In the rat model of CPB, we further demonstrated that CPB induced migration of immature monocytes into the lungs, either from the bone marrow or from the spleen. Moreover, we confirmed the hypothesis that immature subsets could contribute to CPB-induced acute lung injury by giving rise to TNF-alpha producing descendants. Conclusions The immature CD14(low)CD16-monocytes might contribute to blood-circuit contact-induced acute lung injury by generating TNF-alpha-producing, mature monocytes. New strategies based on monocyte manipulation could be a promising therapeutic approach for minimising CPB-related lung injury.