Objective: To observe whether the use of WhatsApp™ can contribute to the improvement and retention of ECG knowledge during medical graduation in a period-time of 4 months. Material and Methods: A controlled, quasi-randomized, intention-to-treat, clinical trial. Medical students of the 2nd and 3rd semester attended a 2-hour class on elementar ECG interpretation. A test with 10 ECG tracings covering subjects was applied with a possible 0-10 score at 4 different times: immediately before the lesson (M1), immediately after the lesson (M2), one month after the lesson (M3) and four months after the lesson (M4). Intervention group, formed by 2nd-semester students, were included, shortly after M2, in a WhatsApp™ group, in which final year medical students and a cardiology resident discussed ECG tracings frequently in the absence of a teacher. Control group, formed by 3rd-semester students, was instructed to study on their own. Results: 13 students were included from the 2nd semester and 11 from the 3rd semester. In M1, the intervention and control group obtained a median of 0.0. In M2, both groups presented a similar increase with a median of 4.0 (IIQ=2.8-5.0) for the intervention group and 4.5 (IIQ= 3.3-5.5) for the control group. In M3, there was a difference between group scores, with a median of 6.0 (IIQ= 3.5-7.0) for the intervention group and 2.0 (IIQ=0-4.0) for the control group (p=0.016). In M4, difference was maintained (4.0 for intervention group [IIQ= 3.0-6.3] vs. 1.0 [IIQ= 1.0-3.0] for control group [p= 0.006]). Conclusion: Early-stage medical students learned and retained more elementar ECG knowledge when participating in WhatsApp™ ECG group discussion with more advanced medical students and medical resident, even without a teacher in this group.