This paper presents findings of a study that verified the relationship between the professional isolation of school principals and their performance at work. A survey of all principals in a suburban area of Quebec (n=166) elicited a 70 percent response rate. The principals completed the UCLA Loneliness Scale and Self-Appraisal Instrument for Community College Administrators. Findings indicate a negative, significant correlation between isolation and performance. The findings could be due to the high proportion of males in the sample; men usually keep their feelings to themselves. Other explanations are that professional isolation generates dissatisfaction toward the work environment, which in turn affects performance; or that professional isolation often results in a lack of support, information, and feedback. One table and one figure are included. (Contains 34 references.) (LMI)