In order to determine the pathogenesis of fever in solid tumors, we studied the association of fever at diagnosis in children with solid tumors (malignant lymphoma, rhabdomyosarcoma, and neuroblastoma), serum levels of interleukin 1 (IL-1), and tumor necrosis factor. Thirteen of 20 patients (65%) with solid tumors were complicated with fever at diagnosis. There was no difference in C-reactive protein or IL-1 levels between the patients with and without fever, while the erythrocyte sedimentation rate and TNF levels were higher in the former than in the latter by Wilcoxon's rank sum test (p less than 0.01). These findings suggest that most febrile episodes at diagnosis in children with solid tumors are associated with the release of tumor necrosis factor.