Results of intervention studies on the effects of α-linolenic acid (ALA; C18 : 3n-3) on blood pressure (BP) are conflicting. Discrepancies between studies may be due to differences in study population, as subjects with increased baseline BP levels may be more responsive. Therefore, we examined specifically the effects of ALA on 24-h ambulatory blood pressure (ABP) in (pre-)hypertensive subjects. In a double-blind, randomised, placebo-controlled parallel study, fifty-nine overweight and obese adults (forty males and nineteen females) with (pre-)hypertension (mean age of 60 (sd 8) years) received daily 10 g refined cold-pressed flaxseed oil, providing 4·7 g (approximately 2 % of energy) ALA (n 29) or 10 g of high-oleic sunflower oil as control (n 30) for 12 weeks. Compliance was excellent as indicated by vial count and plasma phospholipid fatty-acid composition. Compared with control, the changes of –1·4 mmHg in mean arterial pressure (MAP; 24 h ABP) after flaxseed oil intake (95 % CI –4·8, 2·0 mmHg, P=0·40) of –1·5 mmHg in systolic BP (95 % CI –6·0, 3·0 mmHg, P=0·51) and of –1·4 mmHg in diastolic BP (95 % CI –4·2, 1·4 mmHg, P=0·31) were not statistically significant. Also, no effects were found for office BP and for MAP, systolic BP, and diastolic BP when daytime and night-time BP were analysed separately and for night-time dipping. In conclusion, high intake of ALA, about 3–5 times recommended daily intakes, for 12 weeks does not significantly affect BP in subjects with (pre-)hypertension.