This experiment was conducted to determine the effects of Standardized Ileal Digestible (SID) lysine intake during lactation on litter and reproductive performance of gilts. A total of 784 gilts (PIC Camborough) were used in a 21-d lactation. Gilt was considered the experimental unit and all fostering was recorded and occurred within 24 hours post-farrowing. All litters were standardized to 13 nursing piglets within 24 hours of farrowing and prior to gilts starting the trial. Gilts were offered 2.3 kg/d of common gestation diet from entry into farrowing rooms until farrowing day. Gilts were allowed ad libitum access to feed from day 2 post farrowing until weaning. Dietary treatments were: 0.90, 1.03, 1.15, 1.28, and 1.40% SID Lys. Two corn-soybean meal based diets with 3,322 kcal ME/kg were formulated with the lowest and highest SID Lys levels and blended with a computerized system to achieve the intermediate SID Lys levels. L-Lys in the diets ranged from 0.25 to 0.44% and soybean meal ranged from 23 to 38%. Data was fit using the GLIMMIX procedure of SAS© (Cary, NC) with SID Lys treatment as fixed effect and farrowing week and room used as random effects. Amino acid analysis confirmed total formulated values. Gilts weighed 199.7 kg ± 2.13 post-farrowing, which likely contributed to a low overall lactation feed intake of 4.6 kg/d ± 0.11, which may have caused gilts to be energy limited in this study. Increasing dietary lysine showed a reduction in backfat at weaning (linear, P=0.026), a significant increase in total litter weight at weaning (linear, P=0.075), a significant increase in litter weight gain (linear, P=0.057), and a significant improvement in lactation litter gain:feed (linear, P=0.029). In conclusion, increasing lysine intake of gilts during lactation maximized litter growth in this study up to 63 grams of SID Lys intake per day despite an apparent energy limitation. SID Lysine, % SEM Probability, P< 0.9 1.03 1.15 1.28 1.4 Linear Quadratic Sow BW change, kg -5.9 -6.6 -7.1 -5.7 -7.1 1.4 0.668 0.912 Backfat at weaning, cm 1.3 1.28 1.29 1.26 1.22 0.028 0.026 0.408 Litter weight gain, kg/d 2.37 2.38 2.37 2.49 2.44 0.044 0.057 0.993 ADFI, kg 4.68 4.69 4.53 4.65 4.53 0.107 0.526 0.994 g of SID Lys intake, g/d 42.0 48.2 52.2 59.2 63.4 1.08 0.001 0.889 Subsequent total born, n 13.9 14.4 14.6 14.1 14.8 0.37 0.221 0.832 [ABSTRACT FROM AUTHOR]