We investigated the efficacy of fish oils in Portuguese patients with hypertriglyceridaemia and mixed hyperlipidaemia, and the influence of fish consumption on the triglyceride lowering capacity of fish oils. Forty patients participated in this double-blind study, consisting of a 4-week dietary or wash-out baseline period after which patients were randomly assigned to receive either 12 fish oil capsules (3.6 g/day of omega 3) or similar 12 soya oil capsules per day for a period of 2 months. There were no statistically significant changes of total, HDL or LDL-cholesterol, and triglycerides. Nevertheless, triglycerides increased 19.9% with soya oil and decreased 27.8% with fish oils. Also, there was an inverse relationship (rho = -0.352) between fish consumption and fish oils effect on triglycerides, and the triglyceride lowering with fish oils increased (from 27.8% to 44.4%), reaching borderline significance, if we excluded patients consuming one or more meals with fish per day. Glucose increased 11% (P = 0.0047) with fish oils. These findings suggest that the triglyceride lowering effect of fish oils is affected by fish consumption, and confirm that fish oils increase blood glucose levels in diabetics and non-diabetics.