In recent years, several reports shown that specific long chain fatty acids as omega-3 had beneficial effects on fertility of dairy cows. Also, among the fatty acids, the omega-3 fatty acids possess the most potent immunomodulatory activities. The objectives were to test the effects of supplementation of α-linolenic acid (ALA) from flaxseed on yields, health and fertility of dairy cows. The experiment was conducted in a large commercial dairy farm. 516 cows were randomly divided into 2 groups 21 d before the expected calving. The cows in the treatment group (FLX; n = 276) were fed during the dry period a diet that contained 4% (DM basis) of a supplement providing 80 g ALA per cow/day, and postpartum a diet contained 5% of the same supplement providing 220 g ALA per cow/day. The control (CTL; n = 240) cows were fed a diet with similar content. Cows were routinely examined by a veterinarian 7 to 10 d after calving, treated according to the farm's routine management and body condition score (BCS) was determined at that visit and at peak lactation. Production data were analyzed using the PROC MIXED procedure. Treatment was analyzed as a fixed effect whereas the cow and days in milk as random effects. Distributions were analyzed using the PROC FREQ procedure of SAS. Milk yields were 4.5% higher (1.8 kg/d; P < 0.001), and fat and protein percentages were 3.2 and 3.69%, and 3.16 and 3.22%, in the FLX and CTL groups, respectively (P < 0.002). Ketosis incidence was lower in the FLX than in the CTL group (23.5 and 31.2% respectively; P < 0.05). Severe ketosis occurred less in the FLX cows (P < 0.03), and the incidence of severe metritis tended to be less in the FLX cows (P < 0.1). The mortality rate in the FLX was 0.7% compared with 4.6% in the CTL group (P < 0.005). The BCS and body weight were higher in the FLX than in the CTL cows (P < 0.006). No differences were observed in conception rates at 1st or 2nd inseminations, but waste and open days were 17 and 18 d, respectively, lower in the FLX cows (P < 0.07). In conclusion, supplementation of ALA to dairy cows increased milk yield, reduced fat and protein percentages, improved body condition, decreased the incidence of ketosis, severe metritis and mortality rate, and tended to decrease waste and open days.