Serum concentrations of iron, copper, zinc and magnesium and also serum transferrin and ceruloplasmin were investigated in 28 children aged 10 months to 10 years with undue susceptibility to infections. None of the children had any classical immune defect. Seven of them had had frequent upper respiratory tract infections, 16 had suffered from frequent infections of the middle ear and five from mainly lower respiratory tract infections. Thirteen healthy children aged 9 to 18 years residing in the same area served as controls. The children with undue susceptibility to infections had significantly lower mean serum iron (p less than 0.05) and zinc (p less than 0.001) levels than the healthy controls. The mean serum concentrations of copper and magnesium and of transferrin and ceruloplasmin did not differ between the patients and controls. Children with frequent middle ear infections seemed to account for most of the differences in the serum levels of iron and zinc. An inverse correlation was observed between duration of breast feeding and serum concentration of zinc, and between weight as well as height and serum magnesium. The reasons for these changes and the possible role of trace element deficiency as a factor predisposing to or perpetuating undue susceptibility to infections in children are discussed.