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Purpose and Methods: We investigated whether a high-dose chemotherapy regimen of cyclophosphamide 1,800 mg/m2, 4'-epidoxorubicin 60 mg/m2, etoposide 330 mg/m2, and cisplatin 120 mg/m2 given monthly for four cycles with recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) support (5 micrograms/kg daily for 10 days) could improve the survival of patients with extensive-stage small-cell lung cancer (SCLC) compared with a standard-dose regimen (cyclophosphamide 1,200 mg/m2, 4'-epidoxorubicin 40 mg/m2, etoposide 225 mg/m2, and cisplatin 100 mg/m2) given monthly for six cycles. Planned cumulative doses of the drugs were the same in both treatment arms except for cisplatin (which was 80% in the higher-dose plus rhGM-CSF group)., Results: At the time of the preplanned interim analysis, 125 patients, 60 in the standard-dose group and 65 in the higher-dose plus rhGM-CSF group, had entered the study; 116 were eligible, 55 in the standard-dose group and 61 in the higher-dose group. All patients were included in the analyses. The cumulative doses of each drug actually delivered were significantly higher in the standard-dose group. No difference in response rates was observed between the two groups. There were significantly greater hematologic toxicities, documented infections, and transfusions of RBCs and platelets in the higher-dose plus rhGM-CSF group. Patients in this group proved to have a shorter survival duration and a shorter time to relapse than patients in the standard-dose group (median overall survival: standard-dose, 10.8 months; higher-dose, 8.9 months; log-rank test with adjustment for prognostic variables, P = .0005; respective probabilities of relapse at 1 year, 77 +/- 0.6 and 96 +/- 2.2; log-rank test, P = .013)., Conclusion: A 50% increase in dose-intensity for this four-drug regimen could not be achieved with GM-CSF due to excessive toxicity in patients with extensive-stage SCLC.