The purpose of the present study is related to the criterion that should be followed to form animal groups according to BW, and to evaluate the effect of this practice on productive, behavior, or health outcomes when animals are mixed at the onset of fattening. A total of 160 Holstein bulls (162 ± 0.3 kg BW and 124 ± 1.1 d age) were allocated in 8 pens with similar mean BW, and were randomly assigned to 1 of 2 treatments: homogeneity of initial BW that corresponded to a CV of BW within a pen of 7% (HO), and heterogeneity with a CV of BW within a pen of 21% (HE). Concentrate intake and health incidences were recorded daily, straw consumption weekly, and BW every 14 d. Animal behavior was registered on d 0, 1, 3, 5, 8, 14, 28 and every 28 d by scan sampling. Animals were slaughtered after 206 d, and HCW and carcass quality were recorded. Data were analyzed using a mixed-effects model with repeated measures. Performance and carcass data were not affected by the BW uniformity of the groups at the onset. However, while the CV of BW within HO was maintained (P > 0.10) throughout the study (6.1 ± 0.73%), the CV of BW within HE was reduced (P < 0.01) from 21.5 to 10.7 ± 0.86% at the beginning and at the end, respectively. During the 1st month HO exhibited (P < 0.05) lesser completed mounts than HE (0.5 and 1.0 ± 0.09 times/15 min). Throughout the study HO tended (P = 0.10) to perform more self-grooming compared with HE (13.6 and 11.6 ± 0.13 times/15 min). Whereas 80% of veterinary therapies applied in HE were chronic (≥3 consecutive treatments in the same animal), 90% of therapies in HO were acute (≤2 consecutive treatments in the same animal, P < 0.05). Moreover, 6 HE bulls were removed from the study for health reasons. In conclusion, even if bulls from pens with a non-uniform BW at the onset had greater frequency in completed mounts and worse health status than bulls from pens with uniform BW at the onset, animal growth and carcass was not impaired.