Objective: To assess the effects of fish (lean or oily) and fish oil consumption on blood lipid concentration during weight loss. Design: Randomized, controlled 8-week trial of energy-restricted diet varying in fish and fish oil content. Subjects, 324 men and women, aged 20-40 years, body mass index 27.5-32.5 kg m(-2), from Iceland, Spain and Ireland, were randomized to one of four groups: (1) control (sunflower oil capsules, no seafood), (2) cod diet (3 x 150 g week(-1)), (3) salmon diet (3 x 150 g week(-1)), (4) fish oil (DHA/EPA capsules, no seafood). The macronutrient composition of the diets was similar between the groups and the capsule groups were single-blinded. Measurements: Total cholesterol (TC), high-density lipoprotein (HDL) and low-density lipoprotein cholesterol, triacylglycerol (TG) and anthropometrics were measured at baseline and end point. Results: The difference in logTG lowering between the control group and the cod diet, salmon diet and fish oil from baseline to end point was -0.036 (95% CI -0.079 to 0.006), -0.060 (-0.101 to -0.018) and -0.037 (-0.079 to 0.006), respectively. Reduction in TC was about 0.2 mmol l(-1) greater in the fish groups (cod and salmon) than in the control group, but only of borderline significance when adjusting for weight loss. HDL tended to decrease less in the diet groups consuming a significant amount of n-3 fatty acids (salmon and fish oil). Conclusion: Weight-loss diet including oily fish resulted in greater TG reduction than did a diet without fish or fish oil. Controlled trials using whole fish as a test meal are encouraged to be able to elucidate the role of different constituents of fish for human health.