BACKGROUND The toxicity of interleukin-2 (IL-2) administered by continuous infusion has not been investigated in children. METHODS This study enrolled 10 boys and 7 girls with refractory malignancy. Recombinant IL-2 was infused continuously for cycles of 120 hours at doses escalating from 3 to 30 x 10(6) IU/m2 per day. RESULTS A total of 45 cycles was administered. The most common toxicities included fever, anemia, hypotension (usually responsive to fluid bolus), hyponatremia, oliguria, hypoalbuminemia, nausea, thrombocytopenia, vomiting, and weight gain. Most side effects were correlated significantly with a dosage of 18 x 10(6) IU/m2 or more per day. Respiratory distress was infrequent. Two cycles were interrupted due to severe toxicity (hypotension in one case and confusion in another), but there were no fatalities. During infusion, there was a significant nondose-related increase in serum IL-2 receptor levels. Despite this evidence of immunomodulation, no objective tumor response was noted. CONCLUSIONS Continuous infusion of IL-2 at doses up to 30 x 10(6) IU/m2 per day can be administered safely to children by this method.