The objective was to investigate the effects of the β-adrenergic agonist cimaterol (CIM) on growth and carcass quality of Friesian young bulls at different developmental stages. The study comprised three liveweight groups (WG) each of four pairs of monozygotic twins. The average initial liveweight (LW) of the animals in WG 1, 2, and 3 was 162, 299 and 407 kg, respectively. One animal of each pair was fed 0.056 mg CIM per kg LW for 90 days, while the other served as a control (C). The animals were fed concentrate and barley straw according to LW and restricted to 80-90% of the ad libitum level of intake. The animals were slaughtered on day 90 of the treatment period. Without affecting feed intake, CIM treatment increased average daily gain (ADG) by 19% (P<0.001) and thus improved feed efficiency (P<0.001). Dressing percentage, carcass gain, EUROP conformation and longissimus dorsi area were all increased (P<0.001) after CIM treatment. Carcass length (P<0.01), backfat thickness (P<0.05), and internal fat (P<0.001) were reduced after CIM treatment, whereas organ weights were only marginally reduced. Weights of all of 14 different commercial cuts were increased (P<0.002), the increase was largest in cuts from the hind region of the carcass. The percentage of carcass saleable meat increased (P<0.001) in CIM-treated animals. The effects of cimaterol were evident in all three WG. The results show that cimaterol is effective in stimulating overall growth performance and improving carcass leanness of young bulls at different developmental stages, and concomitantly reducing fat deposition in general.