Narrow band imaging (NBI) for detection of blood vessels and microstructures on the mucosal surface is used in gastrointestinal endoscopy since it can improve qualitative diagnosis and detection of lesion. However, there are no studies on flexible hysteroscopy using NBI. We performed flexible hysteroscopy with NBI for outpatients to investigate the sensitivity and specificity of endoscopic diagnosis of malignant endometrial lesions. Of patients who attended our hospital for suspected lesions in the uterine cavity between April 2009 and May 2010, 104 subjects underwent hysteroscopy with NBI, in addition to white light. Using the pathological diagnosis as the gold-standard, we evaluated the sensitivity and specificity of NBI hysteroscopy for detecting atypical endometrial hyperplasia (AEH) or carcinoma. The results were also compared with historical data (n=209) for conventional hysteroscopy using white light only in 2008. The sensitivities were 97.2% [95% confidence interval (95% CI): 90.3-99.7%] and 82.6% (95% CI: 74.4-89.0%) for NBI hysteroscopy and conventional hysteroscopy, respectively. The 95% CIs for the two methods did not overlap and the sensitivity of lesion detection was higher with NBI hysteroscopy. Specificities were comparable, 90.6% (95% CI: 75.0-98.0%) and 85.1% (95% CI: 76.3-91.6%) between the methods. NBI hysteroscopy has increased sensitivity for detection of atypical endometrial hyperplasia (AEH) or carcinoma. A comparison with historical data suggested that NBI may be useful for diagnosis of malignant endometrial lesions. As far as we are aware, this is the first evaluation of flexible hysteroscopy with NBI for diagnosis of malignant endometrial lesions.