To clarify conflicting claims of altered serum concentrations of soluble L-selectin (sCD62L) in recent-onset IDDM, sCD62L was measured in 89 children and adolescents with IDDM (35 recent-onset, 12 during the first year of insulin treatment, and 42 with long-standing (> 1 yr) treatment) alongside 124 controls. Children < 14 yr of age both with and without IDDM (n = 160) had grossly elevated sCD62L concentrations (20.2 +/- 4.9 nmol/l), as compared with adolescents (14-18 yr, n = 23; 15.9 +/- 3.9 nmol/l) and adults (> 18 yr, n = 30; 11.2 +/- 2.3 nmol/l) (p < 0.0001). Multivariate analysis confirmed the strong inverse association between age and sCD62L (p < 0.001) while revealing that sCD62L concentrations were slightly elevated in recent-onset IDDM, as compared with insulin-treated IDDM patients or nondiabetic controls (p = 0.028). Actual sCD62L concentrations in the 35 recent-onset IDDM patients were 22.2 +/- 4.9 nmol/L vs 19.6 +/- 3.6 nmol/l in 35 non-diabetic controls matched for age (p = 0.022). While this significant but small rise of systemic sCD62L reflects leukocyte activation, it is obscured by the inverse association between sCD62L and chronological age in children and adolescents. Therefore, determining sCD62L serum concentrations appears to be of limited value for clinical investigators caring for children and adolescents with IDDM.