Background and study aim Magnifying narrow-band imaging (M-NBI) is useful for the accurate diagnosis of early gastric cancer (EGC). However, acquiring skill at M-NBI diagnosis takes substantial effort. An Internet-based e-learning system to teach endoscopic diagnosis of EGC using M-NBI has been developed. This study evaluated its effectiveness. Participants and methods This study was designed as a multicenter randomized controlled trial. We recruited endoscopists as participants from all over Japan. After completing Test 1, which consisted of M-NBI images of 40 gastric lesions, participants were randomly assigned to the e-learning or non-e-learning groups. Only the e-learning group was allowed to access the e-learning system. After the e-learning period, both groups received Test 2.The analysis set was participants who scored <80% accuracy on Test 1.The primary end point was the difference in accuracy between Test 1 and Test 2 for the two groups. Results A total of 395 participants from 77 institutions completed Test 1 (198 in the e-learning group and 197 in the non-e-learning group). After the e-learning period, all 395 completed Test 2.The analysis sets were e-learning group: n=184; and non-e-learning group: n=184.The mean Test 1 score was 59.9% for the e-learning group and 61.7% for the non-e-learning group.The change in accuracy in Test 2 was significantly higher in the e-learning group than in the non-e-learning group (7.4 points vs. 0.14 points, respectively; P <0.001). Conclusion This study clearly demonstrated the efficacy of the e-learning system in improving practitioners' capabilities to diagnose EGC using M-NBI. Trial registered at University Hospital Medical Information Network Clinical Trials Registry (UMIN000008569).