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Patients receiving high-dose cyclophosphamide as a conditioning regimen for peripheral stem cell collection are subjected over a short period of time to significant exposure to reactive oxygen species (ROS). All these patients undergo profound leucopenia. Various other short-term toxicities are observed in a fraction of the patients, including febrile aplasia requiring hospitalization, thrombocytopenia and mucositis. Although stem cell collection is feasible in the majority of patients stimulated with haematopoietic growth factors, in some instances, graft collection cannot be performed because of insufficient concentrations of stem cells in peripheral blood. There is currently no predictive assay to determine which patients treated with high-dose cyclophosphamide have a high risk of febrile aplasia or will successfully undergo cytaphereses for stem cell collection. In order to identify such predictive factors, we analysed the level of expression before treatment of various ROS detoxification mechanisms in the peripheral blood of 37 patients receiving high-dose cyclophosphamide for lymphoproliferative diseases. Various parameters involved in the metabolism of ROS were measured in plasma and/or erythrocytes, including superoxide dismutase, glutathione, glutathione peroxidase, glutathione reductase and malondialdehyde. High levels of erythrocyte superoxide dismutase before cyclophosphamide therapy were correlated with an increased risk of hospitalization for febrile aplasia (65% vs. 29%, P = 0.013). High superoxide dismutase and low erythrocyte glutathione reductase were associated with lower CD34 yields. These data suggest that components of the ROS detoxification system modulate the degree of short-term toxicity of cyclophosphamide and could be used as predictive markers in individual patients.