There has been limited research on the effects of spatial density on the social interaction of preschool children, particularly those with disabilities. Further, findings of existing studies need to be viewed cautiously due to a number of methodological difficulties including contrived small groupings of children and atypical intervention settings. The effect of varying the spatial density during the regular indoor free play session in a community preschool was investigated in the current study. A single subject alternating treatment design was used to compare the effectiveness of three spatial conditions (small, medium and large) on the social interaction of three children with disabilities or significant language delay. Examination of the findings revealed no consistent pattern of difference in the frequency of peer social interaction in the spatial conditions for two of the target children. There was, however, a modest treatment effect favouring the small condition for a third participant with a language delay. Questions are raised about the efficacy of the intervention for children with significant disabilities in inclusive preschool settings. (Contains 4 tables and 3 figures.)