Triacylglycerol (TG) lowering effects of n−3 long-chain PUFA (n−3 LCPUFA) have been repeatedly demonstrated, but studies investigating the individual effects of EPA or DHA on plasma TG and lipoproteins in man are rare. The effects of a new DHA-rich, almost EPA-free microalgae oil (Ulkenia sp.) on plasma lipids and several safety parameters were investigated in a double-blind, placebo-controlled, parallel design intervention study. Normolipidaemic vegetarians (eighty-seven females, twenty-seven males) consumed daily microalgae oil (0·94g DHA/d) or olive oil (as placebo) for 8 weeks. DHA supplementation decreased plasma TG by 23% from 1·08 (sem 0·07) to 0·83 (sem 0·04) mmol/l (p<0·001). Absolute TG decreases after DHA supplementation were inversely correlated to baseline TG concentrations (r −0·627, p<0·001). Plasma total, LDL and HDL cholesterol increased significantly in the DHA group, resulting in lower TG:HDL cholesterol and unchanged LDL:HDL and total cholesterol:HDL cholesterol ratios. The intake of DHA-rich microalgae oil did not result in any physiologically relevant changes of safety and haemostatic factors. In conclusion, DHA-rich oil from microalgae Ulkenia sp. was well tolerated and can be considered a suitable vegetarian source of n−3 LCPUFA. Although DHA supplementation improved some CHD risk factors (plasma TG, TG:HDL cholesterol ratio), LDL cholesterol increased. Therefore, the overall effects of this intervention on CHD risk deserve further investigation.