OBJECTIVE To study the effect of fish oil supplementation on blood pressure during the third trimester of pregnancy. METHODS In the 30th week of pregnancy 533 healthy women were randomly assigned in a ratio 2:1:1 to receive fish oil (2.7 g/day n-3 fatty acids (Pikasol)), or a control regimen of either olive oil or no oil supplementation. METHODS Blood pressure measured with an automatic device (Dinamap 1846 SX, Criticon) at baseline and in weeks 33, 37, 39 and subsequently weekly until delivery. RESULTS Mean blood pressure increased during the third trimester, and this was not influenced by group assignment. No significant effects on either systolic or diastolic blood pressure were seen in the fish oil group compared to the control groups. The proportions of women with a systolic blood pressure above 140 mmHg or a diastolic blood pressure above 90 mmHg were not significantly different in the fish oil group compared with the control groups, although the proportion of women with diastolic above 90 mmHg tended to be lower in the fish oil group compared with the olive oil group. The corresponding relative risk was RR = 0.48 (95% CI 0.22-1.06; P = 0.07). CONCLUSIONS 2.7 g/day of marine n-3 fatty acids provided in the third trimester of normal pregnancy showed no effect on blood pressure.