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Camborough PIC sows (n = 291) were fed 1 of 5 dietary treatments to evaluate the standardized ileal digestible (SID) Thr:Lys requirement during lactation. Sows were blocked by parity (1, 2, and 3+) and randomly allotted to 1 of 5 SID Thr:Lys ratios (52, 60, 68, 76, and 84). SID Lys was formulated to 1.03% and SID methionine and cystine (M + C):Lys was 55%. Sows were allowed 5.45 kg feed/day starting on the day of farrowing. All other nutrients met or exceeded the NRC (2012) requirements. Sows were allowed ad libitum access to water, and piglets were cross-fostered within treatment within 24 h of birth. Data collected during the study included sow prefarrow BW, sow weaning weight, starting litter weight, weaning litter weight, piglet mortality, weaning-to-estrus interval, and subsequent total born piglets. Data were analyzed as a randomized complete block design using PROC MIXED with sow as the experimental unit and treatment and parity as fixed effects. Requirement estimation models evaluated were quadratic polynomial, broken-line linear, and broken-line quadratic (BLQ) using PROC NLMIXED. Models that differed in their Bayesian information criterion (BIC) values by at least 2 points were considered to have meaningful differences in their data fit. The best-fit model was based on the best BIC value. As the SID Thr:Lys ratio increased, there was a quadratic response on ADFI (4.73, 4.57, 4.67, 4.55, and 4.60 kg/d; P = 0.001) and percent of sow BW loss (-6.4, -7.89, -7.35, -6.19, and -5.81; P = 0.004). In addition, there was a trend for weaning-to-estrus interval to be affected by the ratio (5.4, 4.4, 4.6, 4.5, and 4.8; P = 0.07). Daily litter gain was also quadratically improved as the SID Thr:Lys ratio increased (2.67, 2.67, 2.78, 2.77, and 2.68 kg/d; P-value = 0.001). For daily litter gain, the BLQ model was determined to be the best fit with an estimated SID Thr:Lys requirement of 65%. In conclusion, the optimal SID Thr:Lys ratio for litter growth was 65., (© The Author(s) 2019. Published by Oxford University Press on behalf of the American Society of Animal Science. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)