Abstract: The periparturient period is characterized by negative energy balance because feed intake gradually decreases prior to parturition and increases at a slower rate than milk production. This is one of the contributing factors to occurrence of other peripartum period health problems including mammary gland and reproductive tract infections. Especially high-producing dairy cows are prone to ketosis and infectious diseases, which adversely affect profitability of dairying. To maintain plasma concentration above 3 �g mL -1 at parturition, vitamin E supplementation during the transition period benefits to immune system via improving neutrophil activity and consequently reduces risk for mastitis and retained placenta. Ketone bodies also impair immune function. However, improvement in immune function in cows with ketosis in response to vitamin E supplementation remains to be investigated.