The intake of long-chain n-3 PUFA, including DHA (22:6n-3), is associated with a reduced risk of CVD. Schizochytrium sp. are an important primary source of DHA in the marine food chain but they also provide substantial quantities of the n-6 PUFA docosapentaenoic acid n-6; DPA). The effect of this oil on cardiovascular risk factors was evaluated using a double-blind randomised placebo-controlled parallel-design trial in thirty-nine men and forty women. Subjects received 4g oil/d for 4 weeks; the active treatment provided 1·5g DHA and 0·6g DPA. Active treatment increased plasma concentrations of arachidonic acid, adrenic acid, DPA and DHA by 21, 11, 11 and 88mg/l respectively and the proportions of DPA and DHA in erythrocyte phospholipids by 78 and 27% respectively. Serum total, LDL- and HDL-cholesterol increased by 0·33mmol/l (7·3%), 0·26mmol/l (10·4%) and 0·14mmol/l (9·0%) compared with placebo (all p≤0·001). Factor VII (FVII) coagulant activity increased by 12% following active treatment (P=0·006). There were no significant differences between treatments in LDL size, blood pressure, plasma glucose, serum C-reactive protein, plasma FVII antigen, FVII activated, fibrinogen, von Willebrand factor, tocopherol or carotenoid concentrations, plasminogen activator inhibitor-1, creatine kinase or troponin-I activities, haematology or liver function tests or self-reported adverse effects. Overall, the oil was well tolerated and did not adversely affect cardiovascular risk.