This study was designed to determine whether children with mild mental retardation understand the concept of loneliness, whether their feelings of loneliness at school can be reliably assessed, and whether there are differences in loneliness between children with and without mental retardation. Results from a sample of 62 students with mild mental retardation and 62 students without retardation, ages 8 through 13, indicated that (a) high percentages of both groups understood what loneliness means, (b) a loneliness questionnaire yielded satisfactory internal reliability with both groups, and (c) boys but not girls with mental retardation reported significantly more loneliness than did children without mental retardation. Suggestions were made for future research on loneliness in school settings.