Ten children with locally advanced or recurrent tumors were treated on a Phase I/II study to assess the feasibility and toxicity of intraoperative radiotherapy (IORT) for primary and recurrent pediatric solid malignancies at high risk for local recurrence. Eligible patients include all primary and recurrent pediatric solid tumors that are amenable to resection and have residual microscopic or gross disease after surgery. In all cases, after a gross tumor resection was performed, a flexible, transparent, multichannel applicator was placed and secured within the tumor bed. Once the position of the applicator was optimized, the applicator catheters were attached to the cables of a high-dose-rate remote afterloader, and 1,200 cGy prescribed to 0.5-1.0 cm from the applicator was delivered to the tumor bed via the remote afterloader. One patient with a malignant teratoma developed a peri-rectal abscess 1 month after treatment; no other complications were noted. The 2-year actuarial local recurrence-free and distant metastases-free survival were 80% and 59%, respectively, with a median follow-up of 12 months (range: 3-18 months). The preliminary results suggest that high-dose-rate IORT is a safe and feasible modality for pediatric tumors at high risk for local recurrence. Longer follow-up will be needed to assess fully the toxicity and efficacy of this approach.