PURPOSE: An abdominoperineal resection (APR) has a poor prognosis. However, limited studies about the prognostic factors in APR and the role of preoperative chemoradiotherapy (CRT) have been performed even though in rectal cancer, the application of preoperative CRT provides better local control compared to postoperative CRT. The aim of this study was to identify the prognostic factors and the impact of preoperative CRT in patients who undergo an APR. METHODS: A retrospective analysis was conducted with a total of 133 patients who underwent an APR, cT3, cT4, or cN(+) patients, for rectal cancer between January 1995 and October 2004. Fifty-one patients treated with preoperative CRT (Group 1) were compared with 82 APR patients treated with postoperative CRT (Group 2). Oncologic outcomes were compared between the two groups, and the clinicopathologic factors affecting the treatment outcomes were evaluated. RESULTS: The median follow-up period was 61.2 mo (range 6 to 194 mo). Circumferential margin (CRM) involvement was significantly associated with local recurrence (LR) and with disease-free survival in APR patients (P<0.001, P=0.011). The 5-yr LR rate was significantly lower in Group 1 than in Group 2 (P=0.013) in the univariate analysis, but no difference was noted in multivariate analysis (P=0.315). In Group 1, CRM involvement, tumor size, and lymph node metastasis were significantly lower than they were in Group 2 (P=0.043, P=0.003, P<0.001). CONCLUSION: For achieving adequate oncologic outcomes in APR patients, an adequate CRM should be acquired with an optimal operation. In addition, preoperative CRT would be helpful for high-risk APR patients with a threatening CRM margin, providing the benefit of tumor downstaging.