This study assessed the effect of stocking density on the development of fear reaction (tonic immobility), lameness (gait), and tibial dyschondroplasia in broiler chicks of mixed sex. During 3 experiments, the study used 49,500 broiler chicks (Ross 208). Of these, 432 focal chicks were randomly selected and studied in 3 experiments (144 per experiment). The focal chicks were individually color-marked on the head and wings for identification. Series of stocking densities compared were 9 and 29, 13 and 25, 17 and 21, 18 and 30, and 20 and 28 chicks per m². From 8 to 42 days of age, the study tested focal chicks once a week for lameness (gait) using a scoring scale from 0 to 5 and for the duration of tonic immobility measured in seconds. Each trial terminated with the chicks' being humanely killed and examined for the expression of tibial dyschondroplasia, using a score from 0 to 3. For all series apart from 13 and 25 chicks per m², the average increase in the duration of tonic immobility per day was significantly higher, p < .05, for chicks kept at higher stocking densities. Tonic immobility durations also increased in chicks suffering from tibial dyschondroplasia, especially at 39 days of age, p < .01. The incidence of lameness was higher for high stocking densities among the series compared, p < .05 to p < .001. For most compared series, the growth rate also increased with a unit increase, p < .001. The incidence of tibial dyschondroplasia was significantly higher, p < .01, at stocking densities of 29, 25, 21, 30, and 28 birds per m² compared to those at 9, 13, 17, 18, and 20 birds per m², respectively. A total of 59.6% and 19.2%, respectively, of birds had tibial dyschondroplasia. The results show that high stocking densities adversely affect the welfare of broilers.