A Phase II study of poly(I,C)-LC was performed in 28 children and adolescents with acute lymphoblastic leukemia (ALL), 10 with acute nonlymphoblastic leukemia (ANLL), and 13 with metastatic neuroblastoma. All were refractory to standard chemotherapeutic agents and 25 to an investigational drug. Initial doses of 12 mg/m2 and 9 mg/m2 were intolerable. However, 9 mg/m2 was tolerable in the majority of patients when the drug was started at 3 mg/m2 and increased by 3 mg/m2 increments. Fifteen children with ALL, three with ANLL, and two with neuroblastoma received the drug daily. Seven patients with ANLL and seven children with neuroblastoma received the drug biweekly. Twenty-eight patients received an adequate trial, which was defined as a minimum of 5 weeks at the maximal tolerated dose, unless there was progressive disease at the maximal tolerated dose. Side effects of the drug were striking, and included fever, hypotension, myalgia, bone pain, arthralgia, arthritis, abdominal pain, liver toxicity, thrombocytopenia, and neurotoxicity. No complete remissions occurred in spite of interferon levels above 100 U in nearly 50% of patients.