alpha-Linolenic acid (ALA) is a major dietary (n-3) fatty acid. Some clinical trials with ALA supplementation have shown reduced cardiovascular risk; however the specific cardioprotective mechanism is not known. We studied the effects of daily supplementation with ALA derived from flaxseed oil on concentrations of plasma LDL cholesterol, HDL cholesterol, intermediate density lipoprotein cholesterol, and lipid particle sizes. In a randomized double-blind trial, 56 participants were given 3 g/d of ALA from flaxseed oil in capsules (n = 31) or olive oil containing placebo capsules (n = 25) for 26 wk. Changes in plasma HDL cholesterol, LDL cholesterol, and triglyceride concentrations did not differ between the 2 groups at 26 wk. The adjusted plasma total cholesterol concentration at 26 wk was 0.45 mmol/L higher in the flaxseed oil group (5.43 +/- 0.03 mmol/L) compared with the olive oil group (5.17 +/- 0.07 mmol/L) (P = 0.026). ALA did not affect LDL, HDL, or IDL particle size; however, the concentrations of the large, less atherogenic LDL1 (P = 0.058) and LDL2 (P = 0.083) subfractions tended to be greater in the ALA group. In conclusion, ALA does not decrease CVD risk by altering lipoprotein particle size or plasma lipoprotein concentrations.