Use of Mohs micrographic surgery to establish quantitative proof of heightened tumor spread in basal cell carcinoma recurrent following radiotherapy. We compared 27 basal-cell carcinomas (BCCs) recurrent following radiotherapy and subsequently excised by Mohs micrographic surgery to a control group of BCCs recurrent following other treatment modalities and similarly excised. Mohs technique permitted precise, quantitative tumor assessment, obtained via a novel method utilizing three parameters: the number of surgical stages required for complete excision, the percentage increase between clinical preoperative tumor area and final postoperative defect area, and the presence of deep subcutaneous tissue invasion. Figures for the postradiation group were larger in all three categories, with the latter two revealing statistically significant differences versus the nonradiation group. This study gives strong, direct quantitative support to the clinical impression that BCC recurrent following radiotherapy is a uniquely aggressive, invasive subset of recurrent BCC. 