Using 73 Large White gilts, we studied the lysine requirement during lactation. Sow and pig performance and nitrogen balance of sows were measured during the total 21-d lactation period. Variation in body composition of sows between farrowing and weaning was determined by the comparative slaughter technique. Two levels of crude protein (15.5 and 17.1%) were combined with two levels of L-lysine HCl supplementation (0 and .16%), resulting in four diets termed P1L1, P1L2, P2L2, and P2L3, containing .66, .77, .77, and .87% crude lysine, and 15.5, 15.5, 17.1, and 17.1% crude protein, respectively. Sows' feed intake, litter growth rate, and milk energy and protein output were not affected by the diet. Nitrogen balance over the total lactation increased (P < .001) with an increase in lysine and(or) protein (−11.7, −7.0, −3.4, and .4 g N/d for P1L1, P1L2, P2L2, and P2L3 diets, respectively). The losses of body weight and muscle weight were higher in P1L1 gilts (P < .01 and .05, respectively) than in gilts on other treatments, whereas the loss of adipose tissue and backfat was similar for the four groups. Multiple linear relationships between lysine intake and milk nitrogen output as predictors and nitrogen balance (N balance = −15.8 + 1.22 lysine − .63 N milk, R² = .89) or variation of muscle weight (Vm = −477 + 30.1 lysine − 14.7 N milk, R² = .70) as predicted values were calculated to estimate lysine requirement according to production performance of sows. Calculations show that for achieving a zero protein balance, 45 to 55 g/d of crude lysine are required for normal to high-yielding sows, respectively.