OUTLINE: Patients receive a continuous infusion of high-dose methotrexate IV over 24, 30, 36, or 42 hours depending on time of study entry. Beginning at hour 42 or 48, patients receive leucovorin calcium IV every 6 hours for 3 days or until plasma methotrexate concentration is < 0.2 µM. Treatment repeats every 2 weeks in the absence of disease progression or unacceptable toxicity.