Information was collected on the environmental opportunities for exercising self-determination among 281 adults with mental retardation receiving community-based residential supports. The results indicated that: (1) the majority of participants had little or no opportunity to exercise self-determination over major life decisions (e.g., with whom and where to live, the recruitment and retention of care staff); (2) even in more mundane areas, such as where and when to eat, the majority of participants were not supported to exercise effective control; (3) variation in environmental opportunities to exercise self-determination was strongly related to a range of factors including participant ability, previous residential history, and structural and procedural aspects of the residential supports currently provided.