Patients receive vincristine IV on day 2 of the first course (day 1 of subsequent courses) and days 8, 15, 22, and 29, and irinotecan IV over 1 hour on days 1-5 and 22-26. Courses repeat every 6 weeks in the absence of disease progression or unacceptable toxicity. Patients with responsive or stable disease receive additional courses of therapy for a maximum of 1 year.