Although thiazolidinediones and magnesium supplementation improves insulin action and increases HDL-cholesterol, the potential link between serum magnesium and thiazolidinediones has received little attention. Focusing on the increase of serum magnesium, 63 eligible subjects were enrolled and randomly allocated to receive either 30 mg Pioglitazone once daily (Group A) or lifestyle intervention (Group B) during 12 weeks. Subjects were eligible if they were glucose-intolerant, and excluded if they had high blood pressure, diabetes or abnormal liver function tests. The personnel assessing outcomes were blinded to group assignment. Of the 63 eligible subjects, 3 dropped out (one in group A, and two in Group B) because they moved out of the city. So, 30 subjects in each group, who satisfactorily completed the follow-up, were included in the analysis of data. There were no serious adverse events or side effects due to Pioglitazone or lifestyle intervention. At baseline, the groups did not differ significantly in serum magnesium levels 1.73 +/- 0.17 versus 1.72 +/- 0.14 mg/dl, p = 0.80. Subjects who received Pioglitazone significantly increased their serum magnesium to 1.93 +/- 0.16 mg/dl whereas in the lifestyle intervention group the increase was 1.74 +/- 0.25 mg/dl, p < 0.0001. What this study showed was a significant increase in the serum magnesium levels of glucose-intolerant subjects who received 30 mg Pioglitazone once daily.