The effects of reducing CP level in combination with an increase in ambient temperature (29 vs 22 degrees C) on performance and carcass composition were studied in a factorial arrangement of treatments involving 66 Piétrain x (Landrace x Large White) barrows from 27 to 100 kg BW. Animals were fed at each temperature one of three experimental diets that provided 0.85 or 0.70 g of digestible lysine per megajoule of NE, in the growing (27 to 65 kg) and the finishing (65 to 100 kg) phases, respectively. Diet 1 was a corn, wheat, and soybean meal diet formulated without crystalline AA; CP levels were 20.3 and 17.6% for the growing and the finishing phases, respectively. In Diets 2 and 3, CP level was reduced by substituting part of the soybean meal with corn and wheat (Diet 2), or with corn, wheat, and 4% fat (Diet 3). Diets 2 and 3 were supplemented with AA and balanced according to the ideal protein concept. The CP levels of Diets 2 and 3 were, respectively, 15.8 and 16.3% in the growing phase, and 13.4 and 13.8% in the finishing phase. Pigs were housed individually and had free access to feed and water. The ADFI was measured daily, and animals were weighed weekly. Carcass composition was measured at slaughter (100 kg BW). Increasing ambient temperature from 22 to 29 degrees C resulted in a 15% reduction in ADFI and 13% lower ADG. Leaner carcasses (P < 0.01) were obtained at 29 degrees C (22.8 vs 24.8% carcass fat). At 22 degrees C, ADFI was lower (P < 0.05) for the low-CP diets, but daily NE intake, ADG, and carcass composition were not affected (P > 0.05). At 29 degrees C, ADFI was not different (P > 0.05) between diets and daily NE intake was higher (P < 0.05) with Diet 3 than with Diet 1, and the difference was more important during the finishing period than during the growing period. Using the model ADFI = a BWb, estimates of b were 0.65, 0.53, and 0.53 at 22 degrees C and 0.50, 0.44, and 0.50 at 29 degrees C, for Diets 1, 2, and 3, respectively. The higher NE intake for Diet 3 at 29 degrees C did not improve ADG (P > 0.05) but increased mainly fat deposition. These results indicate that a 4 percentage unit reduction of dietary CP level reduces N excretion (minus 37%) but does not affect growth and carcass composition as long as the ratio between essential AA and NE are kept optimal. In addition, diets with reduced CP limit the effect of high ambient temperature on ADFI. Finally, our results demonstrate the significance of using NE, rather than DE or ME, for formulating diets.