Limited research has been conducted to determine the optimum dietary amino acid balance for finishing pigs. In a nitrogen balance study, lysine utilization in finishing pigs fed corn and soybean meal-based diets (CSBM) improved when the standardized ileal digestible threonine to lysine ratio was increased from 0.80 to 0.89. In a subsequent study, growth performance and carcass quality were similar in finishing pigs fed a 14% CP CSBM diet and a 10% CP CSBM diet, fortified with synthetic amino acids. In the 10% CP CSBM diet, the standardized digestible threonine, methionine, methionine plus cysteine, tryptophan and isoleucine to lysine ratios were 0.72, 0.35, 0.66, 0.21 and 0.71, respectively. The addition of synthetic threonine to the 10% CP CSBM diet resulted in a reduction in growth performance. It is speculated that the response of finishing pigs to added threonine is influenced by environmental conditions and by effects on intestinal microflora.