In this study a comparison between the classical (111In oxine) and the newer (99Tcm HMPAO) technique of labelling leucocytes is reported. The behaviour in vivo and the relative sensitivity in the detection of infection (chest and bone) and inflammatory bowel disease (IBD) is presented. Simultaneous dual-radionuclide gamma camera acquisition methodology was applied to study 99 patients, 18 with chest infection, 26 with bone infection, 41 with IBD and 14 with other pathological conditions. The mean (1 SD) 50% washout time from the lungs was 483.03 (79.10) s for 99Tcm HMPAO-labelled white blood cells and 475.85 (83.79) s for 111In oxine-labelled white cells (r = 0.81). Concordance between the two techniques was 94% in the chest-infection group of patients, 88% in the bone-infection group and 71% in the localization of IBD.