Eighty crossbred pigs of a composite genotype were allocated at 80 kg liveweight to a 2 × 5 factorial experiment involving 2 sexes (boars and gilts) and 5 levels of dietary energy intake ranging from about 55% estimated ad libitum up to 100% ad libitum intake. The diet was formulated to be protein-adequate and contained 14.4 MJ DE/kg and 0.55 g available lysine/MJ DE and the pigs were slaughtered at approximately 120 kg liveweight. Growth rate and food conversion efficiency increased linearly, in response to increasing digestible energy (DE) intake, with boars consistently out-performing gilts at each level of DE intake. Protein deposition rate in the whole empty body of pigs was consistently higher in boars than in gilts and linearly related to DE intake in both sexes, with no evidence of a plateau at high energy intakes, suggesting no intrinsic limit to protein deposition in these pigs up to 120 kg liveweight. When pigs were offered the protein-adequate diet ad libitum between 80 and 120 kg liveweight, boars and gilts consumed 47.7 and 40.9 MJ DE/day, respectively, and protein deposition rates in the whole empty body of pigs reached 247 and 182 g/day, respectively. Maintenance energy requirements were estimated to be 351 kJ DE/kg^0.75.day for both boars and gilts [maximal model; including nominal DE level (treated as a 5-level factor), actual DE intake (treated as a continuous variable), and sex (treated as a 2-level factor)] or 506 and 566 kJ DE/kg^0.75.day for boars and gilts, respectively [reduced model; including actual DE intake (treated as a continuous variable) and sex (treated as a 2-level factor)]. Statistical analyses show that the maximal model reflects the data more closely than the minimal model, suggesting that the lower maintenance estimate of 351 kJ DE/kg^0.75.day could reflect reality better. In conclusion, genetically improved boars, and to a lesser extent gilts, have a high capacity for lean growth, and their upper limit to protein retention might not be reached below about 120 kg liveweight.