Type 2 diabetes mellitus (DM) is being diagnosed more frequently in children and adolescents. Thailand has a low incidence of childhood DM. This study reviewed patients with DM in the Division of Pediatric Endocrinology, Faculty of Medicine, Siriraj Hospital compared to our previous study. The results demonstrate that type 2 DM in Thai children and adolescents has increased from 5% during 1986-1995 to 17.9% during 1996-1999. Mean age was 11.6 years. Mean BMI was 27.8 kg/m2. Fifty-six percent were diagnosed on routine examination. The period of increase in type 2 DM is associated with an increase of obesity prevalence from 5.8% in 1990 to 13.3% in 1996. This result emphasizes the importance of encouraging daily physical activity and healthy diet in our populations and also alerts our pediatricians and endocrinologists to the possibilities of type 2 DM in these age groups.