Low serum magnesium levels are related to diabetes mellitus (DM) and high blood pressure (HBP), but as far as we know, there are no previous reports that analyzed the serum magnesium concentration in individuals with metabolic syndrome (MS). We performed a cross-sectional population-based study to compare 192 individuals with MS and 384 disorder-free control subjects, matched by age and gender. Magnesium supplementation treatment and conditions likely to provoke hypomagnesemia, including previous diagnosis of diabetes mellitus (DM) and/or high blood pressure (HBP), were exclusion criteria. In this regard, only incident cases of DM and HBP were included. MS was defined by the presence at least of two of the following features: hyperglycemia (> or =7.0 mmol/l); HBP (> or =160/90 mmHg); dyslipidemia (fasting triglycerides > or =1.7 mmol/l and/or HDL-cholesterol <1.0 mmol/l); and obesity (body mass index > or =30 kg/m(2) and/or waist-to-hip ratio > or =0.85 in women or > or =0.9 in men). Low serum magnesium levels were identified in 126 (65.6%) and 19 (4.9%) individuals with and without MS, p<0.00001. The mean serum magnesium level among subjects with MS was 1.8+/-0.3 mg/dl, and among control subjects 2.2+/-0.2 mg/dl, p<0.00001. There was a strong independent relationship between low serum magnesium levels and MS (odds ratio (OR)=6.8, CI(95%) 4.2-10.9). Among the components of MS, dyslipidemia (OR 2.8, CI(95%) 1.3-2.9) and HBP (OR 1.9, CI(95%) 1.4-2.8) were strongly related to low serum magnesium levels. This study reveals a strong relationship between decreased serum magnesium and MS.