Transforming growth factor-beta1 (TGF-beta1), a multi-functional cytokine, is involved in regulating a variety of cellular activities and the serum/plasma TGF-beta1 level is altered with various diseases. However, most published reports have described adult patients, and so we investigated the clinical significance of serum TGF-beta1 level in pediatric patients. The diagnostic application of the measurement of serum TGF-beta1 level depends critically on the control value, however, there is no information on the control value of serum TGF-beta1 for children. In the present study, we determined the serum TGF-beta1 level of healthy Japanese children as a control value with enzyme-linked immunosorbent assay (ELISA). The serum TGF-beta1 level of children (0-14 years old) was significantly higher than that of adults (over 15 years old) (p < 0.01). Thus, it is recommended that when the serum TGF-beta1 levels of patients are evaluated, they should be compared with those of age-matched controls.