During the transition period, cows exhibit reduced immune cell numbers and function, and elevated markers of inflammation. Compromised immune function is thought to be caused, in part, by metabolic stress and by changing hormone concentrations due to the transition from a pregnant state to a lactating state. If not properly managed, the transition period is accompanied by increased incidence of periparturient diseases. Feeding polyunsaturated fatty acids (PUFA) has been shown to affect immune function in dairy cattle. The objective of this study was to determine the effects of feeding a flaxseed supplement enriched in omega-3 PUFA on immune function, milk yield and components, and FA composition of milk, plasma, and red blood cells. Multiparous Holstein dairy cows (n = 15) were randomly assigned to 2 treatments: control-fed cows (n = 8) received whole roasted soybeans at 4.8% DM, and flaxseed-fed cows (n = 7) received an extruded flaxseed product (LinPRO-R; O&T Farms) at 3.5% DM. The diets contained similar concentrations of crude protein and fat. Diets were fed for the first 21 d of lactation. Blood was collected on d 1, 7, 14, and 21 and milk on d 7, 14, and 21. Milk fat percentage tended to be greater (P = 0.07) in the flaxseed group (4.5%) compared with the control group (3.9%). Flaxseed-fed cows tended to have increased α-linolenic acid in milk (P = 0.06) and in plasma (P = 0.09) compared with controls. Neutrophil expression of reactive oxygen species was reduced in flaxseed-fed cows (P < 0.01) and phagocytosis also tended to be reduced (P = 0.08). There was a tendency for decreased mRNA abundance for tumor necrosis factor (P = 0.09) and interleukin 10 (P = 0.09) in peripheral blood mononuclear cells in flaxseed-fed cows. In summary, feeding an extruded flaxseed product increased ALA in plasma and milk without reducing milk fat or protein percentage or yield. Cows fed the omega-3 diet had reduced reactive oxygen species and phagocytosis activity ex vivo.