Intensive chemotherapy prior to harvesting autologous bone marrow may be advantageous for cancers that infiltrate the bone marrow. This approach, however, may deplete hemopoietic reserves or damage the hemopoietic microenvironment and thereby jeopardize posttransplant engraftment; in vitro treatment of bone marrow to ensure a tumor-free state may enhance the risk of nonengraftment. We addressed these concerns in a pilot study of patients with disseminated neuroblastoma. Bone marrow was harvested after initial intensive therapy that included 1) three to six courses of very high dose cyclophosphamide (CPM) (100-140 mg/kg in five patients, 140-160 mg/kg in nine patients), plus doxorubicin and vincristine, followed by 2) a median of three strongly myelosuppressive courses of cisplatin/VP16. The median interval between courses was 23 days. The marrow was treated in vitro with the CPM congener 4-hydroperoxycyclophosphamide (4-HC) and cryopreserved. It was infused after a massive cytoreduction regimen of melphalan 180 mg/m2 (n = 7) or thiotepa 900 mg/m2 (n = 7), plus cisplatin, BCNU, VP16, and local radiation. All 14 patients reconstituted hemopoiesis (median of 37 days to neutrophils greater than or equal to 500/microliter and of 63 days to platelets greater than 50,000/microliters). The number per kilogram body weight of hemopoietic progenitor cells (CFU-GM, BFU-E) in the autografts correlated significantly with the rate of hematologic recovery. Preharvest CPM dosage, however, did not. The use of thiotepa, as opposed to melphalan, was associated with a significantly slower posttransplant platelet recovery, suggesting a possible adverse effect of high-dose thiotepa on posttransplant hemopoiesis. In sum, while reinforcing evidence of a stem cell sparing effect of CPM, this pilot study more importantly delineates the degree to which high-dose alkylator therapy can be exploited to attain maximal dose intensity beginning at diagnosis. This aggressive treatment approach may have wide applicability, since CPM is among the most active agents against a spectrum of pediatric cancers.