BACKGROUND: There is no proven medical therapy for plexiform neurofibromas (PNs). We undertook a phase II trial of pegylated interferon (PI) to evaluate response and time to progression (TTP). METHODS: PI was administered as a subcutaneous injection to patients with neurofibromatosis type 1related PN, stratified by the presence of symptoms (asymptomatic: stratum 1, symptomatic: stratum 2) or documented imaging progression (stratum 3). Patients in strata 1 and 2 received PI for up to one year if stable, 2 years for those with clinical (stratum 2) or imaging response (>/=20% decrease in volume). Patients on stratum 3 continued PI until progression. PI was considered active in stratum 3 if TTP doubled compared with the placebo arm of a previous randomized trial using tipifarnib. RESULTS: Enrolled were 82 evaluable patients (median age 10 y; range 1.6 to 21.4). Fatigue and/or worsening of behavioral issues were the most common toxicities requiring dose modification. Across all strata, imaging responses were seen in 4 patients (5%). Three of 26 symptomatic patients on stratum 2 met the criteria for clinical response without corresponding imaging changes. In stratum 3, median TTP was 29.4 months versus 11.8 for the placebo arm of the previous trial (P=.031). The slope of tumor growth on PI slowed significantly compared with the slope before starting PI (P=.044). CONCLUSIONS: In patients with active PN, PI results in more than doubling of the TTP compared with placebo. Imaging changes in symptomatic patients were not associated with changes in clinical status.