Diets with high inclusion of corn fermented protein (CFP) may have an excess of Leu, which has a negative impact on the growth performance of pigs. Tryptophan is a precursor of serotonin, which is a neurotransmitter that is critical for feed intake regulation, and excess dietary Leu may reduce synthesis of serotonin in the brain by preventing Trp from being transported to the brain. Reduced serotonin synthesis can result in reduced feed intake and pigs with reduced feed intake due to excess Leu also have reduced growth performance. As a consequence, if diets are formulated based on CFP instead of soybean meal (SBM), diets need to be fortified with extra Trp to account for the reduced efficiency of Trp due to the excess Leu in corn protein. However, a quantitative assessment of how much extra Trp is needed in diets based on CFP instead of SBM has not been conducted. Therefore, an experiment was conducted to test the hypothesis that the optimum Trp:Lys ratio in diets for growing pigs is greater if CFP rather than SBM is used as the protein source in the diets.