Over the last 30 years the trend on dairies has been to increase the energy content of the pre-partum ration to enhance dry matter intake (DMI) during the final weeks of gestation and prepare the rumen for the higher energy diets of lactation. In the last few years a number of dairies have shifted to a completely different dietary strategy; feeding a lower, but adequate, energy diet, which utilizes straw as a major component of the diet. The objective of this study was to determine if the institution of a lower energy pre-partum diet had any negative effects on metabolic or immunologic status of cows when compared to cows fed a higher energy pre-partum diet. Primiparous heifers were assigned to either a HIGH E diet (1.56 Mcal NE(L) / kg) or a LOW E diet treatment (1.35 Mcal NE(L)/kg diet) about five wks before calving. DMI was 1.2 kg/d greater in the HIGH E cows in the three weeks prior to calving. The LOW E diet did reduce the magnitude of DMI depression just before calving, but apart from a small reduction in plasma non-esterified fatty acid concentraion (NEFA) the day of calving, few major benefits of this reduction were seen. DMI after calving was similar in the two groups of cows. Body condition scores of the cows just before calving and at 21 and 44 days in milk (DIM) were statistically similar across the dietary treatments. A glucose tolerance test was performed on a subset of animals in each dietary treatment about one week prior to calving. No statistically significant difference was observed in glucose clearance from the blood or insulin secretion pattern suggesting no difference in tissue sensitivity to insulin was induced by the dietary treatments. Milk production during the first 45 days of lactation was statistically similar in HIGH E (27.1 kg/d) and LOW E cows (28.2 kg/d). Dietary treatment had no significant effect on liver triglyceride content on day 1 after calving, days to first ovulation, or plasma NEFA profile after calving. In vitro immune function was assessed by neutrophil iodination, differential white blood cell counts, and whole blood interferon- production in response to both recall antigens and non-specific stimulation. There was no evidence that prepartal dietary energy impacted the degree to which these in vitro tests of immune function were suppressed in the transition cow. It is suspected that prepartum energy would have an effect on colostrum quality, but in this study, prepartum dietary energy had no effect on protein and IgG content of colostrum. The competency of the immune system as a whole was assessed by response to intramammary challenge with a mildly pathogenic strain of Escherichia coli. Throughout the course of the challenge, quantitative milk bacterial culture, milk somatic cell count, rectal temperature, differential WBC counts, serum minerals, and acute phase response were assessed and no differences were noted between dietary treatments. All animals were similarly able to successfully recover from the experimentally induced E. coli mastitis. Feeding a high energy diet prior to calving offers no advantages to health or production over a lower energy diet. The utilization of a low energy density diet to limit energy intake pre-partum is not detrimental to the dairy heifer.