The behavior of 304 mentally disabled adults was observed in five settings (one residence, four sheltered workshops) during periods when they were free to affiliate with peers. Regression analyses using settings, personal traits (age, sex, IQ, and diagnosis), and mediating variables (e.g., physical attractiveness, desire for affiliation, and length of institutionalization) were conducted to predict various aspects of affiliative behavior. Settings accounted 16 to 63 percent of the predictable variation independent of personal and mediating variables. Although older and mentally ill clients affiliated less extensively, neither degree of retardation, length of previous institutionalization, use of medication, or other physical disabilities appeared to affect affiliation independent of other variables. In general, clients who were physically attractive desired affiliation, and had intelligent peers in their programs affiliated more extensively and intensively with peers. In total, the findings indicate that the variables most predictive of affiliation in the present community settings were also the ones most amenable to personal or environmental change.