While 18F-FDG labelling of monocytes would allow in vivo trafficking with positron emission tomography (PET), present methods suffer from poor retention of radioactivity. We investigated the feasibility of utilizing insulin for improved [18F]fluorodeoxyglucose (18F-FDG) labelling. Separated human monocytes and lymphocytes were labelled with 18F-FDG with or without 3 h insulin pre-incubation. Insulin had no effect on lymphocyte labelling (21.4+/-0.8% vs 20.8+/-1.1% efficiency, P=NS). However, for monocytes, insulin pre-incubation led to a 169+/-9% increase in labelling efficiency (19.3+/-4.1 vs 32.5+/-1.8, P<0.05), without significant effects on cell activation or viability. Moreover, while only 57.7+/-4.8% and 40.4+/-5.6% of the 18F-FDG was retained at 1 and 3 h for controls, the retention rate increased to 91.6+/-2.1% (P=0.01) and 86.5+/-1.9% (P<0.01) after insulin pre-incubation. Improved 18F-FDG retention was accompanied by a 70.3+/-7.4% decrease in glucose-6-phosphatase activity (P=0.02). PET imaging of rats showing hepatic ischaemia-reperfusion injury demonstrated higher liver uptake for monocytes labelled after insulin treatment. Thus, insulin improves monocytic 18F-FDG uptake and retention, and may provide a feasible labelling method for PET imaging.