We evaluated the efficacy, toxicity and feasibility of two cycles of high-dose chemotherapy (HDC), each supported with mobilized peripheral blood progenitor cells (PBPC). Ninety-six patients with metastatic or high-risk cancers received disease-specific HDC regimens. The first cycle consisted of cyclophosphamide 6000 mg/m2, thiotepa 500 mg/m2 and carboplatin 1200 mg/m2. Alternatively, some patients received etoposide 1800 mg/m2 substituted for thiotepa. A second cycle was planned 6-8 weeks later and consisted of mitoxantrone 60 mg/m2 with either melphalan 140 mg/m2 or thiotepa 600 mg/m2. PBPC were mobilized with either growth factor alone or cyclophosphamide followed by growth factor(s). Thirty-four of 96 enrolled patients (35%) did not receive the second cycle. The reasons were: patient refusal (15); insurance refusal (three); toxicities of cycle 1 (seven); no response to cycle 1 (four); and inadequate mobilization or poor engraftment, (five). Of the 33 patients who entered cycle 2 with measurable disease, 28 demonstrated further response after cycle 2 (including 10 who entered CR). One patient died of toxicity after each cycle. Hematologic recovery was rapid and complete in most patients. Tandem cycles of high-dose chemotherapy supported by PBPC are feasible and safe, although many patients fail to receive the second treatment. Preliminary evaluation shows evidence of further antitumor efficacy following cycle 2.