Lowering the CP level in piglet diets reduces the risk of post-weaning diarrhea and N excretion to the environment. The question remains at what point CP becomes limiting. An experiment was designed with 2 standardized ileal digestible (SID) Lys levels (10 and 11 g) and 6 CP levels (140, 150, 160, 170, 180, 190 g/kg) in a 2 x 6 factorial design (with 6 pens of 6 animals each per treatment). Linear and quadratic (QP) mixed models of performance in function of CP were fitted to study the effect of Lys and CP and their interaction. To determine optima, QP models and broken line models with linear (BLL) or quadratic (BLQ) ascending portions were fitted through the data. It was hypothesized 1) that the response to a decreasing digestible CP level could be described with broken line models and 2) that the breakpoint of these models is dependent on the dietary SID Lys level. Decreasing the CP level decreased ADG (P < 0.001). For G:F, the effect of decreasing CP level depended on the SID Lys level (P of the interaction = 0.028 in the linear model and P = 0.002 in the QP model). According to the BLL model, with 11 g SID Lys in the diet, G:F started to decline with CP levels below 176 g CP (SID Lys:apparent total tract digestible (ATTD) CP = 0.077), and with 10 g SID Lys, CP levels below 165 g/kg (SID Lys:ATTD CP = 0.075) depressed performance. Serum creatinine levels showed a linear decrease with increasing Lys:CP levels (P < 0.001). Across both SID Lys levels, when fitting a BLL model, minimal serum urea levels were reached at a Lys:CP ratio of 0.064. This seems to be the point where CP and not Lys limits muscle deposition. The small difference in breakpoint between serum urea level and performance suggests that the composition of nonessential AA may be also at stake. The effect of decreasing CP level depends on SID Lys and using a maximal SID Lys:CP ratio may be useful for optimizing the AA profile of dietary CP. When the Lys:CP ratio exceeds 0.064 (SID Lys:ATTD CP above 0.079), protein and not individual AA limits growth in most piglets between 4 and 9 weeks of age.