Serum eosinophil cationic protein (ECP), but not serum myeloperoxidase (MPO), has been found to reflect disease activity of asthma and eczema, but no reference values exist for young children. Thus, we aimed to provide values of serum-ECP and serum-MPO in young children without obstructive airways disease (OAD), and determine possible influencing factors. Parental interview was performed and serum was collected from a total of 245 children (207 children aged 24-41 months and 76 children aged 0-23 months) with no history of lower respiratory disease. Repeated serum samples were obtained in 38 subjects. Ten percent of the children had active eczema at examination. All children were controls in the "Environment and Childhood Asthma" study in Oslo. Geometric means (GM +/- 1.96 SD) for serum ECP were 11.8 microg/l (2.5-56.0) and 7.9 microg/l (2.0-30.4), respectively, in the 0-23 and 24-41-month-old children, with the corresponding values for serum MPO 453 microg/l (153-1349) and 347 microg/l (142-859), respectively. Age was inversely associated with serum-ECP and serum-MPO, most pronounced in the youngest children. Active eczema and maternal daily smoking adversely affected serum-ECP, but not serum-MPO. Gender and parental atopy did not influence the results. We conclude that serum-ECP in very young children is influenced by age and active eczema and is related to maternal smoking in a dose-dependent fashion. These factors should be considered when assessing inflammatory markers in very young children.