The paper presents a case report of a 6-year-old boy with broad QRS tachycardia, who was accidentally diagnosed with a cardiac tumour. The tachycardia occurred twice, 10 months apart, and was quickly interrupted in a night medical care unit by performing a carotid sinus massage. Electrocardiography was performed, but its low quality did not allow for a detailed analysis, except for the heart rate, which was 180 bpm. Cardiological diagnosis was performed in a reference centre. No laboratory abnormalities were found. Holter ECG recorded only 374 single ventricular beats, while a routine echo revealed a very large 4.2 × 3.1 cm tumour in the interventricular septum, which did not impede intracardiac blood flow. The presence of the tumour was confirmed by magnetic resonance imaging of the heart, in which a fibroma was suspected. A beta-blocker (metoprolol) was included in the treatment and further cardiac monitoring was recommended. The boy was discharged home with a recommended follow-up in 2 months. He did not report for the appointment, while his parents requested for full imaging documentation.