To determine the apparent diffusion coefficient (ADC) value and clinical-pathologic risk factors associated with postoperative local recurrence or distant metastases in patients with rectal cancer.This retrospective study was approved by our institutional review board and written informed consent was waived. Sixty-one consecutive patients with rectal cancer (41 men and 20 women; mean age, 64.5 ± 12.1 years; range, 32–86 years) underwent pelvic magnetic resonance (MR) imaging. Tumor ADC value, clinical-pathologic factors were compared by using Kaplan-Meier method and Cox proportional hazards model.Of the 62 lesions in the 61 patients, 12 (19.4%) had postoperative local recurrent or distant metastasis during a median follow-up period of 38.5 months. In the multivariate analysis, lymphatic invasion (hazard ratio [HR] = 34.70; 95% confidence interval [CI] = 5.97, 201.54; P < 0.0001) and pathological N stage (HR = 0.19; 95% CI = 0.06, 0.61; P = 0.0018) were independent pathological risk factors for the prediction of postoperative local recurrence or distant metastases. Plasmatic CA19-9 level (HR = 1.05; 95% CI = 1.02, 1.09; P = 0.0027) and tumor ADC value (HR = 0.01; 95% CI = 0.00, 0.28; P = 0.0076) were independent, preoperatively obtained, and non-invasive risk factors.In our study, multivariate analysis demonstrated plasmatic CA19-9 level (≥ 37 U/ml), pathological N stage (>N2), lymphatic invasion (> ly2), and the ADC value (< 0.996 × 10^-3 mm²/sec) were the significant risk factors for the prediction of postoperative local recurrence or distant metastases in the patients with rectal cancer. In these risk factors, preoperatively predictable factors were only plasmatic CA19-9 level and the ADC value for postoperative local recurrence or distant metastases. In our study population, pathologically proven TNM stage was not a significant risk factor which is a quite opposite result from what is expected. Previous studies reported that pelvic MR imaging was useful for the evaluation of tumor aggressiveness, extramural depth of tumor invasion, and extramural vascular invasion ^1-4. Especially, the ADC value was widely used for the evaluation of tumor aggressiveness and correlated with worse prognostic factors, including pathological T stage, plasmatic CA19-9 level, Ki-67 labeling index, and tumor differentiation grade ^2,5. The ADC value might be useful as an imaging biomarker which is non-invasive assessment of the entire tumor and reflects cellularity and water content. Malignant tumor contains some interstitial region such as inflammatory cell infiltration, fibrosis, interstitial edema, tumor necrosis, and mucin. These pathological factors may affect the diffusion of water molecules resulting in the decrease of ADC values ^6-7. Tong et al. reported that the ADC value had a significant correlation with extramural depth of tumor invasion in rectal cancer. Authors concluded that the tumor with lower ADC value was associated with more advanced extramural depth of tumor invasion resulting in poorer prognosis ³. We believe that the ADC value may reflects these tissue components and tumor aggressiveness or disease-free survival which is demonstrated in this study. Traditionally, postoperative chemoradiotherapy (CRT) was recommended for the patients with pathological T3 and/or N1-2 tumors ⁶. However, postoperative CRT is considered for the patients with high risk of postoperative local recurrence or distant metastases (involved margins, poorly differentiated grade, and lymphovascular invasion) if preoperative radiotherapy has not been received ⁷. In our study, in addition to pathological T or N stage and lymphatic invasion which were previously described as the risk factors for postoperative local recurrence or distant metastases in the patients with rectal cancer, the ADC value was also revealed as a “new” risk factor. Therefore, we believe that the patients with preoperative ADC value can be an additional evaluating factor for the indication of postoperative CRT, or at least for the recommendation of close follow-up. Our study had several limitations. First, this study was a retrospective study with a relatively small sample size, which might have potentially caused selection bias. Second, the ADC value measurement was not fully represent the overall tumor. Third, we did not evaluate the patients received neoadjuvant chemotherapy or radiotherapy. Finally, the median follow-up period of 38.5 months was relatively short. Further clinical studies with lager sample size and longer follow-up period need to be performed to validate our quantitative data.The plasmatic CA19-9 level and tumor ADC values were useful for the preoperative prediction of high risk cases for postoperative local recurrence or distant metastases in the patients with rectal cancer.