The objective of the current study was to determine if group-housing Holstein heifer calves in indoor pens with poor ventilation and drainage influences performance, health, leukocytes, and behavioral responses compared with individually housed calves. Ninety colostrum-fed calves (2±1 d of age) were randomly assigned to 3 treatments: individually housed (G1; n=30 calves), 2 calves per pen (G2; n=30 calves), or 3 calves per pen (G3; n=30 calves). The space allowance per calf was 2.5m2 for all treatment groups. All calves were fed 747 and 1,010g/d of dry matter of a 28% CP and 20% fat milk replacer during the first 2wk and wk3 to 6, respectively. Weaning was initiated on d 46 by removing the evening feeding, and calves were completely weaned when they consumed 800g/d of dry matter calf starter for 2 consecutive days after d 54. Calves were randomly commingled at d 90 in groups of 5 calves per pen in outdoors pens with an attached hutch. Peripheral blood was collected during the neonatal (3, 10, 21d), weaning (46, 48, 54d), and commingling periods (90, 93, 98d) and was analyzed for neutrophil oxidative burst capacity when cocultured with an Escherichia coli, neutrophil surface L-selectin protein concentration, and whole blood secretion of tumor necrosis factor-α when cocultured with lipopolysaccharide. Behavior of each calf was assessed using 2 independent tests: an approach response to a human subject and the response of the calf when placed in an isolated cage. Calf starter intake was greater for G2 and G3 during wk8 and 9, and also at wk 11 for G3 compared with G1. No treatment × time interaction or treatment effect for average daily gain was observed. Additionally, no treatment × time or treatment effect was noted for any leukocyte or biochemical variable of biological significance throughout the entire study. Individually housed calves tended to have a reduced incidence of respiratory disease during the first 90d of life. No other treatment differences for the other health outcomes were observed. Group-housed calves also relied more on calf-to-calf interactions than calf-to-human interactions, as evident by their reduced approach of the human observer and more frequent movement when placed in an isolated pen. Group housing in an environment with poor ventilation and drainage may increase the risk of respiratory disease.