Thirty patients with relapsed pediatric solid tumors received high-dose carboplatin and etoposide with autologous marrow support in a dose-escalation trial. These patients had received extensive prior treatment, which included both cisplatin and etoposide in 25 cases. Six patient cohorts received carboplatin in doses of 1200-2100 mg/m2 and etoposide in doses of 960-1500 mg/m2. All courses were associated with severe neutropenia and thrombocytopenia. The median times from bone marrow infusion to granulocyte recovery (> 0.5 x 10(9)/l) and platelet recovery (> 50 x 10(9)/l) were 33 and 28 days, respectively, with similar findings for all dosage levels. The frequency of non-hematologic toxicities was generally low, although hyponatremia (Na+ < 129 mEq/l) was seen in one-third of the courses. Hepatoxicity was dose-limiting and was significantly associated with the cumulative prior cisplatin dose (p = 0.006). There were four toxic deaths (CNS hemorrhage, alfa-streptococcal sepsis, Candida sepsis, and enterocolitis). Eleven patients received a second course of therapy; toxicity profiles and times to hematologic recovery were similar for the two courses. Clinical responses were observed at all dosage levels. Eleven of 26 evaluable patients achieved a clinical response (one complete, 10 partial). The majority of responses were in patients with neuroblastoma (six of 16) or Hodgkin's disease (two of three). For phase II clinical trials, we recommend dosages of 2100 mg/m2 of carboplatin and 1500 mg/m2 of etoposide for children with prior cumulative cisplatin exposure < 960 mg/m2. This carboplatin dose represents a three- to four-fold increase over pediatric doses tolerated without bone marrow support.