Objective: The measurement of serum advanced glycation end products (S-AGEs) in children, adolescents, and young adults with diabetes to determine whether increased S-AGE levels may be associated with long-term glycemic control and early microvascular complications. Study design: The study was performed in (1) 178 children and adolescents with type 1 diabetes mellitus (age range, 2 to 21 years, onset before the age of 12 years; duration longer than 2 years) without clinical and laboratory signs of microvascular complications, (2) 39 adolescents and young adults (age range, 16.1 to 28.8 years) with background or preproliferative retinopathy or persistent microalbuminuria, and (3) 98 healthy age- and sex-matched control subjects. Results: S-AGEs were significantly increased in preschool and prepubertal children with diabetes and were particularly elevated in pubertal subjects with diabetes compared with control subjects. S-AGEs were markedly increased in adolescents with early microvascular complications compared with both control subjects and diabetic patients without retinopathy or nephropathy. No correlation was found between S-AGEs and albumin excretion rate or blood pressure values. Glycated hemoglobulin values and S-AGEs were significantly correlated (r = 0.32; P < .01). In children with poorly controlled diabetes (HbA₁ c >10%), long-term (2 years) improvement of glycemic control resulted in a significant reduction of S-AGE levels in preschool and prepubertal children, as well as in pubertal individuals. Conclusions: S-AGE concentrations may be elevated even in preschool and prepubertal children with diabetes; this means that the risk of microvascular complications may be present at an early age. Improvement in glycemic control may be associated with a significant decrease in S-AGEs. (J Pediatr 1999;134:486-91)