We investigated the effect of 12 months' HMG-CoA reductase inhibitor treatment on plasma polyunsaturated fatty acid concentrations in 19 patients with hyperlipidemia. Arachidonic acid concentrations were significantly increased following treatment (from 110.1 +/- 20.4 mg/l to 129.2 +/- 31.6 mg/l, P < 0.05). The ratio of eicosapentaenoic acid to arachidonic acid was significantly decreased at the end of 12 months' treatment (from 0.702 +/- 0.370 to 0.541 +/- 0.204, P < 0.05). These results suggest that HMG-CoA reductase inhibitors may increase the synthesis of metabolites from arachidonic acid in patients with hyperlipidemia, and that the addition of fish oil is more effective for the prevention of coronary heart disease than HMG-CoA reductase inhibitors alone.