Level of psychological differentiation, a measurable dimension of personality structure, has previously been found to distinguish various pathological “symptom” groups from matched control groups. In the present study, perceptual tests reflecting differentiation were administered to obese and control patients to determine the existence of a significant difference between these groups. Thirty-four obese female volunteers from a nutrition clinic and 34 matched controls within the normal weight range were given a battery of three perceptual tests reflecting differentiation. Results showed the obese group to be significantly less differentiated than the control group.