The addition of supplementary fat to pig diets above current dietary recommendations has generally been viewed in a negative manner because it is considered that the surplus energy will be deposited into adipose tissue, and may therefore be detrimental to carcass value. The current study sought to investigate the effects of adding supplementary fat, in place of starch, to increase the energy density of male and female pig diets on growth performance and carcass quality. A total of 288 Large White × Landrace male and female pigs (boars and gilts) were randomly allocated to pens of 12 pigs each. The effects of (i) sex (gilts v. boars), (ii) added dietary fat level (as tallow; 0 v. 40 and 80 g/kg) and (iii) within added dietary fat level (40 v. 80 g fat/kg), were tested for a 5-week period (Period 1) (n = 4). These diets were formulated to contain 13·5, 14·3 and 15·2 MJ digestible energy/kg for 0, 40 and 80 g added fat/kg, respectively; the calculated standardized ileal digestible lysine content was kept constant at 8·8 g/kg of diet. Thereafter the experiment was designed to test for the same effects over a second 5-week period (Period 2), during which pigs from the initial three treatments were offered the diets with 0 and 40 g added fat/kg. Treatment effects were also assessed over the whole 10 weeks of the study. There was a positive linear effect of dietary fat supplementation in Period 1 on average daily weight gain, whereas dietary fat level in Period 1 had no effect on performance in Period 2, overall, or on carcass weight and P2 backfat thickness (i.e. thickness of fat at 65 mm down the left side from the midline, at the level of the head of the last rib) at 10 weeks. In contrast, supplementation of the diet with 40 g fat/kg during Period 2, regardless of the diet offered in Period 1, increased ADG in the second period and over the entire experiment. There were minimal effects of supplementary fat on feed intake in the entire experiment; however feed conversion ratio (FCR) was influenced by sex over the entire study and tended to be reduced by dietary fat supplementation in Period 2. The diet with 40 g added fat/kg fed in Period 2 increased carcass weight by an average of 2·8 kg and P2 backfat thickness by 1·2 mm. Collectively, the data from the current study suggest that dietary fat supplementation is more effective in finisher pigs than in grower pigs in terms of growth rate, FCR and carcass weight.