The purpose of this study was to determine if there were significant differences in the cognitive dimensions—differentiation, abstractness, and articulation-of illness concepts of school age children and adolescents. A questionnaire was administered to 152 well school age children and adolescents to obtain written concepts of a familiar illness (common cold) and an unfamiliar illness (diabetes). Subjects were 5th, 8th, and 11th graders. Two-way analyses of variance (grade x sex) showed a significant main effect for grade on scores of illness concepts on all cognitive dimensions. With increasing age children's illness concepts were more abstract and articulated.