Thirty entire male Iberian (IB) pigs of 12 kg initial BW were used to study the effects of dietary protein-to-energy ratio on performance, body protein accretion, and tissue composition. Two nutritional regimes were supplied at 3 stages of growth (Phase I, from 12 to 30 kg BW; Phase II, from 30 to 45 kg BW; and Phase III, from 45 to 100 kg BW): the first regime (RefP/E) was formulated to provide optimum protein-to-energy ratios for castrated IB pigs at several stages of growth from 10 to 100 kg BW; the second (IncP/E), for which an overall increase of 2.5 g digestible protein/MJ ME was provided, would allow an increased potential for lean tissue growth presumably well above that expected for the entire male of an obese pig breed. The pigs were surgically castrated at 45 kg BW and slaughtered at about 100 kg BW. In Phases I and II, in which feed was provided ad libitum, no significant differences between dietary treatments in ADG (580 ± 10 and 740 ± 11 g, on average, respectively), G:F (0.475 ± 0.004 and 0.362 ± 0.005, on average, respectively), and gain:ME intake ratio (37.8 ± 0.4 and 26.1 ± 0.4 g/MJ, on average, respectively) were found ( > 0.05). After surgical castration, for the entire fattening period (Phase III), ADFI was fixed and highly restricted. A lower gain:ME intake ratio (14.0 vs. 15.1 g/MJ; 0.05). In contrast, in the fattening stage, similar daily rates of N retention ( > 0.05) were found, in spite of the observed differences in digestible N intake ( < 0.001). We concluded that the protein (lysine) requirements of the IB boar growing from 12 to 30 kg BW and from 30 to 45 kg BW are met with the supply of 11.5 and 10.2 g digestible protein of ideal AA pattern/MJ ME (0.80 and 0.71 g digestible lysine/MJ ME), respectively. As surgical castration is programmed to be abandoned in European Union by 1 January 2018, this information will be important for the efficient production of the immunologically castrated IB pig, at the earlier stages of growth.