Haemopoietic growth factors (HGFs) are being administered to patients with neutropenic fever; however, little is known about the endogenous HGF response in these patients. Specific assays were used to study four HGFs, granulocyte (G-) CSF, granulocyte-macrophage (GM-) CSF, macrophage (M-) CSF and interleukin (IL-) 6 levels in the blood of patients with neutropenic fever (46 episodes). For comparison, levels were also measured in three control populations: normals (20), afebrile neutropenic (14), and bacteraemic but not neutropenic patients (20). In febrile patients, levels of G-CSF (median, range) (0.46, < 0.10-142 ng/ml). IL-6 (0.054, 0.005-24.3 ng/ml) and M-CSF (18.5, 9.9-79.1 ng/ml) were elevated compared with afebrile subjects (< 0.10, < 0.10-1.62 ng/ml). (0.008, 0.002-0.024 ng/ml) and (6.45, < 5.0-31.3 ng/ml) respectively. GM-CSF was not elevated (< 0.02, < 0.02-8.0 ng/ml) compared with afebrile subjects (0.021, < 0.02-0.20 ng/ml). Variables significantly associated (P < 0.05) with elevated cytokine levels were determined by multiple regression analyses. Factors associated with G-CSF elevation were fever, neutropenia, pathogen type and raised bilirubin and creatinine. In contrast, neutropenia was not associated with IL-6 elevation although there was an association between IL-6 elevation and fever, Gram-negative and fungal infections and raised creatinine and bilirubin. M-CSF elevation was associated with fever, renal impairment and known pathogen. Elevated G-CSF and IL-6 levels normalized rapidly (hours-days) with the resolution of infection, whereas M-CSF concentrations remained elevated for up to 10 d. Cytokine levels remained elevated in septic neutropenic patients who did not recover. In summary, G-CSF, IL-6 and M-CSF levels were significantly elevated in sepsis. In contrast, GM-CSF levels were not elevated. These studies should assist the development of therapeutic strategies using HGFs in the treatment of sepsis.