Prevalence of underweight and obesity were investigated in 282 mentally retarded persons living on the West Coast of Norway. Data collected in this survey suggest that people with severe mental retardation were more likely to be underweight and people with mild mental retardation were more likely to be obese. Compared to persons of average intellectual levels, persons with mental retardation were found to be over-represented with both underweight and obesity. Food refusal and self-induced vomiting was more present among persons regarded as underweight and reduced control of food intake was related to obesity. Findings suggest that behavioral variables may have explanatory value with respect to both underweight and obesity. Further investigation using functional analysis of behavior is recommended.