Valine is among the limiting AA in low protein diets for pigs, but data are currently lacking for empirical requirement studies. Therefore, 2 growth assays and 1 N-balance trial were conducted to determine the Val requirement of weaned pigs (8 to 25 kg of BW) expressed as the ratio of standardized ileal digestible (SID) Val to Lys. In Exp. 1, 48 pigs (individual housing; equal proportion of castrates and females; initial BW = 8.0 kg) were randomly subdivided into groups receiving 6 dietary treatments. Dietary SID Val:Lys ratio was increased by adding L-Val from 0.56 to 0.84% (planned increments of 0.06%) at an average SID Lys level of 0.99%. Other crystalline AA were supplied to meet optimum SID AA:Lys ratios based on the concept of an ideal protein. All diets were isocaloric (13.6 MJ of ME/kg) and isonitrogenous (17.8% CP). For the 35-d period, ADG and ADFI increased linearly (P< 0.01) and quadratically (P < 0.01) with increasing SID Val:Lys. However, the data were not suitable for estimation of Val requirement. Therefore, in Exp. 2 the dietary basal level of Val was reduced to 0.49% SID Val and 4 increments of 0.04% and 1 increment of 0.08% L-Val were used to give 6 dietary treatments (48 pigs; individual housing; equal proportion of castrates and females; initial BW = 7.9 kg). The SID Lys level averaged 0.99%, and SID Val:Lys ratios ranged from 0.49 to 74%. Other crystalline AA were supplied to meet optimum SID AA:Lys ratios. All diets were isocaloric (13.7 MJ of ME/kg) and isonitrogenous (17.7% CP). For the 34-d period, ADG, ADFI, and G:F increased linearly (P < 0.01), and ADG and G:F quadratically (P < 0.01) as the SID Val:Lys ratio increased. Estimates of optimum SID Val:Lys were 66, 67, and 61% for ADG, ADFI, and G:F, respectively. These estimates were confirmed by minimized concentrations of plasma urea and Lys and by markedly increased concentrations of plasma Val. To verify the estimates of the growth trials, a total of 24 N balances were conducted with 12 pigs using the dietary treatments of Exp. 2 (Exp. 3; equal proportion of males and females; average BW = 14.1 kg; 2 balances per pig). Pigs were restrictively fed 3 times daily. Precollection and collection periods lasted 7 d each and were repeated after rearranging the animals to treatments. Increasing the dietary SID Val:Lys ratio linearly increased N retention (P < 0.04) and quadratically decreased plasma concentrations of urea (P < 0.01). Optimal SID Val:Lys ratios of 65 and 62% were estimated for N retention and N utilization, respectively. We conclude that an optimum SID Val:Lys ratio of 65 to 67% is needed for optimal performance in 8- to 25-kg pigs.