OBJECTIVE At present, the clinical data about the effect of overall treatment time (OTT) on local control for nasopharyngeal carcinomas (NPC) patients mainly derived from conventional radiotherapy (CRT). The effect of OTT on local control for NPC patients treated with IMRT is still unclear. This study was to explore the effect of OTT on local control in IMRT for NPC patients. METHODS Clinical data of 850 NPC (T1-4N0-3M0) patients that had undergone radical radiotherapy with IMRT from May 2001 to January 2008 in Sun Yat-sen University Cancer Center were analyzed retrospectively. All patients were divided into two groups, which were group with OTT ≤ 42 and group with OTT > 42 days respectively. Survival was calculated using the Kaplan-Meier method. The log-rank test was used to compare survival curves. The effect of clinical factors and treatment related factors on LCR were studied with univariate and multivariate analyses using logistic regression. RESULTS The 5-years local recurrence-free survival (LRFS) rate of group with OTT ≤ 42 were 90.7%, and 90.9% in group with OTT > 42, no significant differences were found between these two groups (χ² = 0.028, P = 0.866). Further stratified analysis found that the LRFS rate for early T-stage patients was no significant difference between group with OTT ≤ 42 and group with > 42 days, they were 97.2% and 97.9% (χ² = 0.672, P = 0.412). For advanced T-stage patients, the LRFS rate of OTT ≤ 42 and > 42 days were 86.5% and 87.2% respectively (χ² = 0.151, P = 0.698). The 5-year LRFS rate were 94.4% vs 93.0% (χ² = 0.090, P = 0.764) at OTT > 42 vs ≤ 42 days for patients treated with IMRT alone, and 89.7% vs 87.6% (χ² = 0.060, P = 0.807) for patients in combination chemotherapy with IMRT. We divided all patients into three groups: OTT ≤ 42 d, 43 - 49 d and ≥ 50 d, the 5-years LRFS rate of the three group was 90.7%, 91.7% and 88.4%, respectively, there was no significant difference of LRFS among those three groups (χ² = 0.136, P = 0.934). Univariate analysis showed that T-stage and GTV volume were correlation with local control. In multivariate analysis, GTV volume was confirmed as independent prognostic factors for local control. CONCLUSIONS Within the range of the OTT observed in our study, prolonged OTT did not have adverse effect on local control. GTV volume was independent prognostic factors in local control for NPC patients treated with IMRT.