The purpose of this study was to evaluate in a phase I-II trial whether low doses of recombinant human interleukin 2 (rHuIL-2) over a prolonged period of time are safe and effective in eradicating or controlling minimal residual disease in children with neuroblastoma given high-dose chemotherapy (HDCT) and autologous stem cell transplantation (ASCT). From January 1992 to July 1996, 17 consecutive patients, with either stage IV or relapsed neuroblastoma, were enrolled. Patients received rHuIL-2 after a median time interval (min-max) of 105 days (56-153) after HDCT and ASCT. The protocol consisted of 2 'priming' courses of rHuIL-2 at escalating doses administered intravenously at 72-h intervals, followed by 'maintenance' with 11 monthly and six bimonthly boosting 5-day courses administered subcutaneously on an outpatient basis. At April 1997, 7 out of the 17 patients had completed the treatment schedule, four had discontinued treatment because of toxicity and four because of relapse; the remaining two patients are still on treatment, having completed 15 courses. Expansion of T lymphocytes, together with an increase in both natural killer cells and in activated T lymphocytes was evidenced. After a median (min-max) follow-up time of 30 (16-64) months, 12 out of 17 patients are alive and well. Two patients relapsed and died 14 and 35 months after transplant. Three patients are alive after having relapsed at 41, 21 and 13 months. The actuarial 2-year event-free survival and overall survival are 67% and 92% respectively. Intermittent administration of low doses of rHuIL-2 given for a long period of time is well tolerated and seems capable of controlling minimal residual disease after HDCT and ASCT in children with high-risk neuroblastoma.