PURPOSE: To determine the remission rate and toxicity of mitoxantrone, etoposide, and cyclosporine (MEC) therapy, multidrug resistance-1 (MDR1) status, and steady-state cyclosporine (CSA) levels in children with relapsed and/or refractory acute myeloid leukemia. PATIENTS AND METHODS: MEC therapy consisted of mitoxantrone 6 mg/m(2)/d for 5 days, etoposide 60 mg/m(2)/d for 5 days, and CSA 10 mg/kg for 2 hours followed by 30 mg/kg/d as a continuous infusion for 98 hours. Because of pharmacokinetic interactions, drug doses were decreased to 60% of those found to be effective without coadministration of CSA. MDR1 expression was evaluated by reverse transcriptase polymerase chain reaction, flow cytometry, and the ability of CSA at 2.5 micromol/L to increase intracellular accumulation of (3)H-daunomycin in blasts from bone marrow specimens. RESULTS: The remission rate was 35% (n = 23 of 66). Overall, 35% of patients (n = 23) achieved complete remission (CR), 12% of patients (n = 8) achieved partial remission, and 9% of patients (n = 6) died of infection. Exposure to CSA levels of greater than 2,400 ng/mL was achieved in 95% of patients (n = 56 of 59). Toxicities included infection, cardiotoxicity, myelosuppression, stomatitis, and reversible increases in serum creatinine and bilirubin. In most who had relapsed while receiving therapy or whose induction therapy had failed, response was not significantly different for MDR1-positive and MDR1-negative patients. CONCLUSION: Serum levels of CSA capable of reversing multidrug resistance are achievable in children with acceptable toxicity. The CR rate of 35% achieved in this study is comparable to previously reported results using standard doses of mitoxantrone and etoposide. The use of CSA may have improved the response rate for the MDR1-positive patients so that it was not different from that for the MDR1-negative patients.