Abstract Background:  Midkine (MK), a heparin‐binding growth factor, is a secreted protein and can be detected in a patient's sera. Method:  MK was studied in the sera of 215 children and adolescents without malignant disease using an enzyme‐linked immunosorbent assay in order to determine the distribution of concentrations in a control population for pediatric oncology patients. Tested subjects either underwent surgical procedures or suffered from endocrinological diseases. Results:  Elevated MK levels were found in patients with short stature, diabetes mellitus, obesity, and cleft lip and palate. These patients were subsequently excluded from the “non‐cancer” group. MK serum levels did neither correlate with sex, age, weight or height nor showed a normal distribution (n= 152, range: 0.0–5.58 ng/ml, median: 0.0 ng/ml, mean: 0.26 ng/ml, SD: ±0.61). Conclusion:  MK serum values in children and adolescents are widely spread and not normally distributed. The present results indicate that the MK expression is influenced by many factors apart from cancer, which have not yet been identified.