The incidence of type 1 diabetes mellitus (T1DM) in Japan among those below 15 years of age is reportedly 1.5-2.5 per 100,000; however, population-based data have not been available in recent years. Although the incidence of T1DM in Japan is extremely low compared with that in Caucasians, the vast majority of Japanese children who are diagnosed with diabetes before the age of 10 years have T1DM. The incidence of type 2 diabetes mellitus (T2DM) in childhood is approximately 3.0 per 100,000, with the figure among junior high school children being 3-6 times higher than that among primary school children. The most common type of diabetes in children appears to shift from T1DM to T2DM with advancing age, especially after puberty. The mortality risk of patients with childhood-onset T1DM diagnosed between 1965 and 1979 is 12.9-fold higher than that of the general population. However, due to efforts aimed at improving medical care and ameliorating the effects of diabetes among children and adolescents, the standardized mortality ratio (SMR) has declined markedly over the past 30 years. The progression of complications in T2DM appears to be more rapid than that in T1DM, primarily due to the number of drop-out cases. Therefore, appropriate education and treatment programs are necessary to protect against diabetic complications in the younger generation. The establishment of an ongoing population-based national T1DM and T2DM registry system is critical for identifying risk factors associated with diabetes and its complications.