Multiple sclerosis is a chronic inflammatory and demyelinating disease of the central nervous system, usually seen in young adults. Early onset of multiple sclerosis at age younger than 18 years is called paediatric multiple sclerosis. Unlike adult multiple sclerosis, paediatric multiple sclerosis causes morbidity at earlier ages and often progresses in a relapsing–remitting form. Although fingolimod is an effective drug used as a disease-modifiying therapy agent in relapsing–remitting paediatric multiple sclerosis patients, it can cause dysryhthmia in the early period after first dose. Our first case is a 14-year-old girl with relapsing–remitting paediatric multiple sclerosis patients who was started to take fingolimod treatment. In the fifth hour of the follow-up, asymptomatic bradycardia was seen and the electrocardiogram was consistent with first-degree atrioventricular block. Her rhythm got spontaneously normal after 12 hours. Second case was 13 years old girl. Steroid treatment was started after her first paediatric multiple sclerosis attack. Despite treatment, she had a second attack 2 weeks after the first attack. Therefore, the neurologist switched to fingolimod therapy. Second-degree atrioventriculer block developed after 4 hours from the initiation of therapy. After 8 hours, rhythm regressed to first-degree atrioventricular block then returned to normal up to 13th hours of follow up. The aim of this article is to draw attention to dysrhythmia side effect of fingolimod which can be fatal. Therefore, the clinician must take precautions. Close cardiac rhythm monitoring is mandatory after the initiation fingolimod theraphy.