The present study assesses the effects of an architectural intervention on residential crowding stress and poststressor effects. Residents of long-corridor, short-corridor, and long-corridor-intervention dormitory floors were surveyed and social behavior and space use patterns were systematically observed over a 3-month period. As predicted, although students living in the three environments were initially comparable, residents of the long-corridor floor (40 residents sharing space) reported more crowding and residential social problems over time, whereas short-corridor residents (20 residents sharing space) and modified long-corridor residents (20 residents sharing space) reported fewer of these problems. The results are interpreted in terms of a model of crowding in which architectural features of interior spaces are associated with space use patterns that facilitate or inhibit informal group development and regulation of the frequency of interaction and the amount of privacy. These conditions, in turn, are related to stress and stresslike symptoms.