Serum concentrations of tumor necrosis factor-α (TNFα), interleukin (IL)-Iβ, IL-6, and the acute-phase reactants C-reactive protein (CRP) and serum amyloid A (SAA) were measured on admission in 46 neutropenic children with cancer in 81 episodes of fever.The aim was to find out whether any of these variables would differentiate true bacteremia from feverdue to other causes. In most episodes serum concentrations of TNFα and IL-Iβ were elevated. IL-6 was detectable in 68%, but the serum concentration was elevated in only 15%. SAA proved to be more sensitive than CRP for the early detection of bacteremia. However, not even SAAwas sufficiently accurate at the individual level.We conclude that the cytokine and acute-phase protein levels found were related to the febrile response but did not correlate with documented bacterial etiology.