Thirty-two Large White × Landrace male pigs were used to determine the relationships between the rates of tissue deposition and age, in boars and barrows under 2 housing systems. A 2 × 2 factorial design was used, with the respective treatments being sex (boar or barrow) and housing condition (individually penned system or group-penned system). Surgical castration was performed at 7 days of age. Individually housed pigs were used to provide an estimate of potential growth and were weaned at 10 days into individual cages and provided with supplemental fermented skim milk for 2 weeks. Group-housed pigs were weaned at 24 days of age and reared in group pens of boars and castrates typical for commercial production. Conventional weaner, grower, and finisher diets were provided ad libitum to all pigs from weaning onwards. A Hologic QDR4500A dual-energy X-ray absorptiometer (DXA) was used to determine lean, fat, and bone composition at 4-weekly intervals from 10 until 150 days of age. Over the 20 weeks of the study, boars deposited less fat than barrows (136 v. 179 g/day, s.e.d. = 6.49, P < 0.001). Over the same time frame, boars deposited more lean tissue than barrows when individually penned (490 v. 444 g/day, s.e.d. = 14.4, P < 0.05) but there was no difference in group-penned pigs (464 v. 443 g/day, s.e.d. = 14.4, P > 0.1). This was more pronounced in the final 4 weeks of growth when individually penned boars deposited 200 g/day more lean than barrows (P < 0.001), with no increase in group-penned animals. However, during this growth phase, group penning further increased the fat deposition margin between boars and barrows where boars deposited 90 g/day less fat when individually penned (P < 0.001), but 140 g/day less fat (P < 0.001) when group penned. The results show that the advantages of boars in terms of growth and lean tissue composition are substantially reduced in group-penned situations. However, because of fat deposition, boars retain some advantage over barrows in group-penned systems at liveweights over about 50 kg. These data may aid in the accurate prediction of the nutrient requirements and optimum slaughter weight for barrows.