Feed efficiency in dairy cattle represents the balance between milk production and feed intake. In the past, improvements in efficiency have been achieved through increased production with little emphasis on feed intake. However, dry matter intake (DMI) is a moderately heritable trait in lactating Holstein cows, suggesting that DMI may be altered through genetic selection. When efficiency is defined as milk energy output (MILKE) per unit of dry matter intake (DMI), high and low efficiency cows differ in MILKE but not DMI. However, considerable variation in DMI exists among cows with desirable MILKE. Thus, consideration of alternate definitions and selection strategies that include DMI may contribute to further improvement in efficiency of lactating dairy cows.