Circulating levels of interleukin (IL)-6, IL-8, soluble Fc gamma receptor type III (sFc gammaRIII), mannose-binding protein (MBP), and C-reactive protein (CrP) were assessed among febrile children with cancer and neutropenia. Levels of IL-6, IL-8, sFc gammaRIII, MBP, and CrP were measured in serum from 56 pediatric cancer patients at the time of admission for 121 episodes of febrile neutropenia (88 febrile episodes without identifiable source, 5 clinically documented infections, 20 episodes of bacteremia due to gram-positive and 5 due to gram-negative organisms, and 3 fungal infections). IL-6 and IL-8 levels were higher in patients with either bacteremia due to gram-negative organisms or fungal infections than in patients with febrile episodes without an identifiable source (P < .00001 for each). IL-6 and IL-8 levels were higher in children with bacteremia due to gram-negative organisms than in those with bacteremia due to gram-positive organisms (P = .0011 and P = .0003, respectively). The measured levels of CrP, MBP, and sFc gammaRIII were not useful for identifying the type of infection. These preliminary results show the potential usefulness of IL-6 and IL-8 as early indicators for life-threatening infections in febrile cancer patients with neutropenia.