Diets were formulated using sugar, soya-bean meal and free amino acids to contain 0.1-0.8 g lysine/MJ digestible energy (DE) and offered at three times maintenance to male and female pigs from 20 to 45 kg live weight. Growth responses and retentions of protein, fat, energy and lysine were assessed. Increasing the dietary lysine concentration resulted in significant (P less than 0.001) linear and curvilinear increases in growth rates and decreases in food conversion ratios. There was only a small effect of lysine concentration on total energy retention, but a substantial effect on the partitioning of energy deposition, with increases in the rate of protein deposition and decreases in fat retention. There was no difference in the efficiency of protein deposition between male and female pigs but males responded more to higher lysine concentrations than females (estimated 0.93 and 0.74 g lysine/MJ DE for males and females respectively). Lysine concentration in the protein deposited by the pigs increased linearly and curvilinearly (P less than 0.01) from 5.8 to 6.6 g lysine/16 g N with increasing dietary lysine concentration. There was a linear and quadratic response (P less than 0.001) in retention of ileal digestible lysine, with the minimum retention of 0.16 occurring at 0.1 g lysine/MJ DE and increasing to a maximum retention of 0.73 at a dietary concentration of 0.47 g lysine/MJ DE. The efficiency of lysine retained/ileal digestible lysine intake was 0.86 and the endogenous lysine loss was estimated at 0.94 g/d.