The aim of this study was to determine the effects of space and the feeding system during the fattening period on the stress biomarkers, performance and carcass yield of steers. A total of 48 Hereford steers with a 214.0 kg live weight (LW) were stocked into eight pens (with six animals/pen) according to the pen size and feeding system: 100DS (100 m²/animal and a daily food supply), 100SF (100 m²/animal and self-feeders), 10DS (10 m²/animal and a daily food supply) and 10SF (10 m²/animal and self-feeders). The finishing diet contained 77% whole grain corn, 20% ground grain corn, 2% slow-release urea and 1% mineral premix. Blood samples were collected from each steer 6 days before the start of the fattening period and at 0, 1, 7, 14, 21 and 35 days from the start of the experiment to measure their white blood cell counts and plasma cortisol and glucose. Dry matter intake (DMI) and average daily gain (ADG) were recorded to determine the feed conversion ratio (FCR). The steers were slaughtered at a 370 – 390 kg LW in a slaughterhouse. Hot carcass weight was measured after the removal of kidney pelvic fat, and the carcass yield was calculated (at 7% dressing). The data were analyzed using a general linear model (Tukey's test, α=0.05). No interaction between factors or feeding system effects was detected for white blood cells (p= 0.7386), cortisol (p= 0.6331) or glucose (p= 0.9734). The white blood cell count was higher in the confined animals on days 7 (46.5 vs. 27.4 10^3/ul) and 21 (138.4 vs. 123.4 10^3/ul). The plasma cortisol concentration also increased across 10DS and 10SF on day 21 (>1.5 mg/dl). Treatment effects were not detected for DMI (p=0.0447), ADG (p=0.97373) or FCR (p=0,8404). Allotting space affected the carcass yield, as animals with more space presented higher values (61.7% vs. 60.3%; p= 0.0010). In summary, the comfort conditions during the fattening period may affect animal welfare, even with a similar animal performance. Furthermore, more space being available is important to improving the carcass yield.