Abstract： Objective To investigat the prognostic value of overall treatment time (OTT) for locally advanced nasopharyngeal carcinoma (NPC) treated with intensity-modulated radiation therapy (IMRT).Methods From May 2001 to April 2007, 376 patients with locally advanced NPC treated with IMRT were retrospectively analyzed.All patients were divided into OTT≤45 days group and OTT >45 days group.The treatment outcomes between the two groups were analyzed.Results Between the groups with OTT≤45 days and OTT > 45 days, the 2-year local control rate (LCR) was 94.9% and 93.1% (χ2= 2.83, P > 0.05) for all patients, 96.3% and 98.7% (χ2=2.83, P>0.05) for patients with T3 disease, 92.2% and 83.1%(χ2= 6.30, P < 0.05) for T4, and 93.1% and 97.5% (χ2= 4.69, P = 0.030) when chemotherapy was concurrently administered.The 2-year LCR was 98%, 96% and 93% (χ2= 2.20, P = 0.531) for patients with treatment interruption before, within and after the 3rd week of IMRT, The Cox regression analysis found that OTT was an independent prognostic factor for LCR in T4 disease.The Linear regression showed that the 2-year LCR was decreased by 2.7% per day of delay.Between the groups with OTT≤45 days and OTT >45days, the 2-year estimated disease-specific survival (DSS), distant metastasis-free survival (DMFS) and overall survival (OS) were 84.1% vs.78.7% (χ2= 0.02, P = 0.881), 87.0% vs.86.1% (χ2= 0.85,P = 0.358), and 91.7% vs.92.2% (χ2= 0.06, P = 0.806), respectively.The further stratified analysis found that the DSS, DMFS and OS were similar between the two groups in T3 (83.7% vs.83.2%, χ2=0.07, P=0.798;86.6% vs.85.7%,χ2=0.02, P = 0.898 ; and 93.7% vs.94.8%,χ2=0.03, P=0.862) and T4 disease (81.4% vs.72.3%, χ2= 0.16, P = 0.687 ;82.6% vs.86.9%, χ2= 1.78, P =0.182;and 88.3% vs.87.5% ,χ2=0.60, P =0.438).In multivariate analysis, T-stage and N-stage were the independent prognostic factors for both DFS and OS, and N-stage was the independent prognostic factor for DMFS.Conclusions The prolongation of the overall treatment time decrease the local control of patients with T4 NPC.