AIMS: The mechanism for the association between diabetes and hypomagnesaemia remains uncertain. This study aimed to test whether hypomagnesaemia is present in pre-diabetes, obesity and the metabolic syndrome. METHODS: 1453 adults from randomly selected households from rural Victoria, Australia, attended for biomedical assessment. Serum magnesium concentrations, hypomagnesaemia defined using local laboratory criteria (<0.70mmol/l), and defined by the bottom quintile of serum magnesium concentrations, were compared in different diabetes pre-cursor states including metabolic syndrome using ATP III criteria. RESULTS: The mean serum magnesium was 0.84+/-0.06mmol/l and 25 (1.7%) had a low magnesium. Mean magnesium was lower among those with known diabetes than those with new diabetes, impaired glucose tolerance (IGT), impaired fasting glucose (IFG) and normal subjects (0.79 (0.78-0.81) vs 0.83 (0.81-0.86); 0.84 (0.82-0.85); 0.84 (0.82-0.86); 0.85 (0.84-0.85)mmol/l). After adjusting for confounders, and compared with those without diabetes, hypomagnesaemia was 10.51 (1.37-80.60)-fold more common with new diabetes, 8.63 (2.20-33.90)-fold more common with known diabetes, 6.77 (1.75-26.17)-fold more common among those taking anti-hypertensive medication but with no difference to those with IGT/IFG (0.90 (0.10-8.10)). CONCLUSION: Diabetes is associated with hypomagnesaemia, but not its pre-cursor states.