Two experiments were conducted to determine optimal standardized ileal digestible (SID) tryptophan:lysine ratio for piglets using growth performance and plasma urea N as response criteria. In Experiment 1, piglets were fed 6 diets with 8.5, 9.5, 10.5, 11.5, 12.5 or 13.5 g · kg^–1 SID lysine to estimate lysine requirement. In Experiment 2, piglets were fed 6 diets with 9.2 g · kg^–1 SID lysine, and 0.178, 0.182, 0.196, 0.216, 0.227 or 0.236 SID trytophan:lysine to estimate optimal tryptophan:lysine ratio. The SID lysine requirement was estimated to be 10.2 g · kg^–1. Response criteria measured were unaffected by SID tryptophan:lysine ratio, implying that the diets fed were not deficient in tryptophan. In conclusion, the SID lysine requirement for the piglets was close to the recommendation by NRC (1998; 10.1 g · kg^–1), but not NRC (2012; 12.3 g · kg^–1). The optimal SID tryptophan:lysine ratio of the piglets was either equal to or less than 0.178.