Abstract Disclosure: J. Yoo: None. H. Woo: None. M. Gu: None. J. Kim: None. Y. Lee: None. C. Shin: None. Y. Lee: None. Background: Childhood-onset type 2 diabetes has increased risk of early diabetes-related complications due to accelerated loss of pancreatic beta cell function. However, no second-choice anti-diabetic drug(such as GLP1 receptor agonists and SGLT2 inhibitors), other than metformin and insulin, is approved to be administered in pediatric patients with type 2 diabetes in South Korea. This study aimed to describe microvascular complications (nephropathy and eye disease) of youths with childhood-onset type 2 diabetes, investigate trends of glycemic control of patients and associated factors across transition period, and analyze change in glycemic control after adding 2nd-choice drug. Method: Among the patients who were treated for type 2 diabetes at Seoul National University Hospital since 2001, 84 patients who were diagnosed under age 18 and reached at age 20 or more on July, 2023 were included. Data of HbA1c, diabetes duration, administered medication, and diabetic complications were collected. Changes in HbA1c from 19 to 22 years of age (transition period) were investigated, and sex, age at diagnosis, and glycemic control at age 19 were analyzed as associated factors. Result: The average age of diagnosis and last follow-up was 14 years and 24.9 years respectively, and the median duration of diabetes was 9.6 years. At the last follow-up, complications developed in 28 patients (33.7%), and the prevalence was 25.3% for nephropathy and 20.6% for eye disease. At the occurrence of complications, the median diabetes duration was 6.2 years for microalbuminuria and 11.3 years for ophthalmic complications. During the transition period, HbA1c showed a significant decline from 8.3% to 7.7% (p=0.007). During this phase HbA1c decreased more significantly in the female (vs. male), in the patients with diagnostic age <15 years (vs. >15 years old), and patients with HbA1c 7% or above at 19-year-old visit(vs. < 7%). During entire follow-up, 59 patients (70%) received diabetes drugs other than metformin and insulin. At diagnosis, the proportion of patients treated with insulin accounted for 55%. At last visit, only few patients were maintained with metformin alone, and most patients required insulin or 2nd-choice antidiabetic drug. Analysis of HbA1c changes in 59 patients with the addition of 2nd-choice drug showed a decrease to 8.5% after 1 year (p=0.006) and to 8.1% at the last visit(p=0.017). Conclusion: Diabetic complications were observed in one-thirds of youths with type 2 diabetes at last follow-up in this study. Glycemic control during transition period to adulthood showed improvement, and factors that influence HbA1c improvement included female, younger age at diagnosis, and better glycemic control at age 19. There is a need to lower the insurance age criteria of South Korea for 2nd-choice antidiabetic drugs, considering that glycemic control significantly improved after adding the 2nd-choice drugs. Presentation: 6/1/2024