The correlation of hyperglycaemia with decreased 2-[(18)F]fluoro-2-deoxy- D-glucose (FDG) uptake by tumours in positron emission tomography (PET) imaging has been clearly established. The available data are mainly based on non-diabetic (non-DM) patients exposed to acute hyperglycaemia after glucose infusion, and little is known about the effect of diabetes mellitus (DM) on FDG uptake by tumours. In this retrospective study we performed a comparison of the tumour uptake in 40 DM patients with the tumour uptake in 145 non-DM patients, all with primary lung malignancies. Peak standardised uptake values (SUVs) without glucose correction were calculated for the lung lesions. Mean (+/-standard deviation) blood glucose concentrations were 6.58+/-2.46 mmol/l in the DM patients and 4.39+/-0.89 mmol/l in the non-DM patients. There was no significant difference between tumour SUVs in DM patients (79 lesions), 5.86+/-3.97, and those in non-DM patients (234 lesions), 6.47+/-4.61. There was no significant difference between tumour SUVs in DM patients with blood glucose <7 mmol/l ( n=28, 64 lesions), 5.91+/-3.98, and those in DM patients with blood glucose >7 mmol/l ( n=12, 15 lesions), 5.68+/-4.09. There was also no significant difference between myocardial SUVs in the DM patients ( n=40), 3.28+/-2.75, and in a similar group of non-DM patients ( n=42), 3.30+/-2.24. We conclude that FDG uptake in lung tumours is not significantly influenced by blood glucose levels in diabetic patients whose blood glucose levels are reasonably well controlled.