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In the present study we assess the antitumor effect and circulating stem cells (CSC) mobilizing capacity of high-dose cyclophosphamide (5 to 7 gr/m2, HDCY). This treatment was given to 21 patients with various hematologic malignancies (8 NHL, 5 MM, 4 HD, 3 CML) excluding 1 with neuroblastoma. All were eligible for later autologous blood stem cell transplantation (ABSCT). To reduce the hematologic toxicity of HDCY, GM CSF was simultaneously administered in 5 patients. HDCY produced a response (as defined by a > 50% reduction of previous tumor mass) in 3 out of 12 HD/NHL and 1 out of 3 MM. Patients with CML were not considered to be evaluable for tumor response. Cell collection yields after HDCY varied widely with a range of 1.5 to 169.9 x 10(4)/Kg (median 13.1) CFU-GM and 1.7 to 18.4 x 10(8)/Kg (median 5.8) MNC collected per patient. Hematologic recovery was rapid and sustained with a median of 16 (12-18) days to PMN > 0.5 x 10(9)/L and 14 (11-18) days to Plt > 100.0 x 10(9)/L. Granulocyte recovery was significantly faster after GM-CSF (13 vs 16 days to PMN > 0.5, p = 0.0008). Non hematologic toxicity consisted mainly of nausea and vomiting, but fatal complications occurred in 2 patients, from pulmonary infection in one and from tumor-lysis syndrome in the other. HDCY represents a useful means of increasing collection of CSC, but toxicity is not irrelevant. Whether a similar anti-tumor effect and mobilizing capacity would be offered by single lower intermediate doses of the drug is still to be ascertained.