OBJECTIVES: Effects of eicosapentaenoic acid (EPA)on visceral fat storage and the autonomic nervous system were evaluated by abdominal computed tomography (measurement of visceral fat area) and power spectral analysis of heart rate variability, respectively. METHODS: The parameters of visceral fat area and heart rate variability were compared between the control group (n=74; conventional therapy) and the EPA group (n=91; conventional therapy plus EPA 1800 mg/day) during a 6-month period. The power spectral analysis of heart rate variability [low frequency component (LF), high frequency component (HF)and LF/HF] was performed on 256 sec taken after a 30-minute rest. RESULTS: Systolic and diastolic blood pressures significantly decreased (p < 0.0001 and p = 0.0076, respectively)but heart rate remained unchanged in the EPA group during the 6 months. In the control group, these parameters showed no change. The values of visceral fat area did not alter in either group but body weight significantly decreased in the EPA group (p = 0.0003). A sex difference was noted in the parameter of visceral fat area; in female patients, the change in the parameter was insignificant, but in male patients this tended to decrease from 162 +/- 60 to 152 +/- 65 cm2 (p = 0.0586) during the 6 months. Serum triglyceride decreased significantly in the EPA group (p = 0.0339) but not in the control group. The ratio of LF/HF in heart rate variability significantly decreased in the EPA group (p = 0.0004) and the decrease was especially prominent in male patients. The LF/HF ratio remained unchanged in the control group. This parameter correlates well with visceral fat area, but not with systolic blood pressure. CONCLUSIONS: The oral intake of purified EPA significantly reduced blood pressure without altering heart rate during the 6-month treatment. EPA suppressed sympathetic nerve activity without inducing any parasympathetic nerve activity. The direct anti-sympathetic action of EPA was inferred and its action was found unrelated to blood pressure decrease. In male patients, diminished visceral fat area may be associated with depression of sympathetic nerve activity.