This descriptive study examined the self-esteem of adolescents with diabetes mellitus and leukemia. Participants included 22 adolescents with diabetes mellitus and 33 with leukemia. No significant difference was found between the self-esteem of males and females in either group. After the initial diagnosis, over time a decrease in self-esteem in the leukemia group and an increase in self-esteem in the diabetes mellitus group were observed. The adolescents with leukemia who experienced a relapse had moderate self-esteem. Most of the adolescents with leukemia could not attend school because of their illness. The mean scores of self-esteem were high in the diabetes mellitus group and moderate in the leukemia group. In both groups, the mean scores of self-esteem were compared to depressive affect, daydreaming, psychosomatic symptoms, intensity of discussion, and parental interest (subscales in the Rosenberg self-esteem scale). No significant difference was found between the two disease groups. There was a strong and significant correlation between self-esteem and depressive affect in both groups.