It is difficult to determine the appropriate resection margin of extramammary Paget's disease (EMPD). A high recurrence rate is reported in spite of using Mohs micrographic surgery (MMS), which is performed commonly. Preoperative mapping biopsy is easier to perform than MMS. In Japan, the following method is recommended instead of MMS: well-defined border and margins histologically confirmed by mapping biopsy should be resected with 1-cm margin and ill-defined border with 3-cm margin. This study aimed to evaluate the accuracy of the Japanese guideline and to assess our mapping biopsy method compared with MMS. Preoperative mapping biopsy specimens were obtained beyond the clinical border for at least four directions in each patient. To confirm the presence of residual Paget's cells postoperatively, narrow specimens were obtained along the surgical margin. Retrospective evaluation of 17 EMPD patients was conducted concerning histological spread of Paget's cells and recurrence ratio. There were 86 directions showing a well-defined border, and in 9.3% (8/86), Paget's cells were still observed at 1-cm resection line. On the other hand, there were 21 directions showing an ill-defined border, and unnecessary radical resection was performed in 90% (19/21) of directions with 3-cm resection line. Although postoperative histological examination showed residual Paget's cells in 47% (8/17) of patients and additional resections were not performed, recurrence rate was only 5.9% (1/17). The resection line of EMPD should be based not on clinical features, but on mapping biopsy. Mapping biopsy is equivalent to MMS concerning recurrence rate and, though conventional, is useful method to treat EMPD. © 2013 Japanese Dermatological Association.