Technical Abstract: The objective of this research was to determine the effects of days dry (DD) on lifetime milk, fat, and protein yields and the dry period length that maximizes lifetime yield for each of these traits. Effect of dry period length was determined separately for DD after first through fourth lactations. A further objective was to determine if the optimum DD for lifetime milk yield depends on previous lactation milk yield, days open (DO), somatic cell score (SCS) or age at calving. Field data, collected through DHI, on US Holstein cows first calving between January 1, 1997 and December 31, 1998 were utilized. Lifetime records were computed as the sum of actual lactational yields. Lifetime milk, fat, and protein production can be maximized with 40 to 45 DD after first lactation and 30 to 35 DD after second and later lactations. Shorter or longer dry periods result in lower lifetime production and higher culling rates in the subsequent lactation. Dry period length to maximize lifetime yield does not appear to depend on previous lactation milk yield, DO, or SCS. However, cows that first calve at 2 yr of age or younger should receive a minimum of 45 to 50 DD after first lactation.