The contribution of iatrogenic blood loss through diagnostic testing to the anaemia of critical illness remains controversial. We measured the effect of an arterial line blood conservation device upon blood loss and anaemia in adult intensive care patients. This randomized controlled trial of 160 patients in a major Intensive Care Unit (ICU) compared a blood conservation device (Venous Arterial Blood Management Protection Plus, VAMP Plus system, Baxter Healthcare) (VAMP group) to a standard arterial pressure line set attached to an arterial catheter (control group). The primary outcome measured was the change in haemoglobin concentration (Hb) during each patient's ICU admission and the volume of blood lost through diagnostic testing in ICU was also recorded. Both groups of 80 patients were matched for age, gender, severity of illness (APACHE II), baseline Hb on entry and ICU length of stay. Both groups had a similar (median [range]) change in Hb during ICU admission (VAMP-7 [-84 to +21] g/l; Control -4 [-67 to +40] g/l; P = 0.33). The VAMP patients lost significantly less blood for diagnostic testing while in ICU (VAMP 63 [0 to 787] ml; Control 133 [7 to 1227] ml; P = 0.001). We conclude that the VAMP Plus system significantly reduced iatrogenic blood loss in critically ill patients, but this reduction did not affect the fall in Hb that accompanies critical illness.