In a randomized, single-blinded, controlled study (430 patients aged 25-63 years, 394 males), 214 subjects were administered a magnesium-rich diet and 216 subjects were administered a usual diet for 12 weeks. Age, sex, body weight, hypertension, diabetes, hyperlipidemia, smoking, obesity, diuretic therapy and hypomagnesemia were comparable between the two groups as were laboratory data at entry to the study. The intervention group A received a significantly higher amount of dietary magnesium (1,142.0 +/- 225 mg/day) compared to group B which received the usual diet (438 +/- 118 mg/day). After 12 weeks, there was a significant decrease in total serum cholesterol (10.7%), low-density-lipoprotein (LDL) cholesterol (10.5%) and triglyceride (10.1%) in group A compared to the values at entry to the study; no such changes were evident in group B subjects. HDL-cholesterol showed a marginal mean decrease of 0.8 mg/dl in group B and 2.0 mg/dl increase in group A. However, in hypomagnesemic patients (26 cases) of the intervention group, there was a 10.9% increase in high-density-lipoprotein (HDL) cholesterol in association with a decrease in other lipids. Although a general blood-lipid-reducing effect of a high-fiber, low-cholesterol diet cannot be excluded, dietary magnesium may have contributed to the reduction of total serum cholesterol, LDL-cholesterol, and triglyceride as well as to the marginal rise in HDL-cholesterol. More studies with a longer follow-up are needed in order to confirm the role of magnesium in preventing a decrease in HDL-cholesterol in association with reduction in other lipoproteins.