Objective:To assess the effects of omega-3 (n-3) fatty acid supplementation on blood pressure and plasma lipids in hypertensives treated with diuretics or beta-blockers.Design:Double-blind placebo-controlled cross-over trial consisting of a 4-week run-in phase and two 6-week intervention phases.Patients:A total of 43 patients of either sex taking a beta-blocker only (n = 29), a diuretic only (n = 3) or a beta-blocker plus diuretic (n=11) for hypertension were recruited from general practice. One patient from the latter group was withdrawn.Methods:Seated blood pressure was measured every 2 weeks in the clinic with a Dinamap. After the run-in phase, participants were randomly assigned to take a supplement of either Omacor (85% n-3 fatty acid concentrate) or corn oil (four 1-g capsules/day) for 6 weeks, after which they crossed over to the other supplement. Fasted blood samples were taken at the end of each phase for lipid analysis.Main outcome measures:The within-individual differences in systolic and diastolic pressure and plasma lipids between Omacor and corn oil treatment.Results:Systolic/diastolic blood pressures measured during the run-in phase were normal (132±2/76±1 mmHg, n = 42) but decreased further with n-3 fatty acid supplementation. The mean within-individual difference in blood pressure compared with corn oil supplementation was 3.1 ± 1.0/1.8±0.6 mmHg (P<0.01). This was accompanied by a 21% reduction in plasma triglycerides (P< 0.01) and a 15% increase in high-density lipoprotein-2 cholesterol (P< 0.01) but there were no significant differences in total or low-density lipoprotein cholesterol.Conclusion:The antihypertensive and hypotriglyceridaemic effects of n-3 fatty acid supplementation seen in the present study suggest that it may be a useful adjunct to antihypertensive therapy with beta-blockers or diuretics.